[api-minor] Replace PDFDocumentProxy.getStats with a synchronous PDFDocumentProxy.stats getter

*Please note:* These changes will primarily benefit longer documents, somewhat at the expense of e.g. one-page documents.

The existing `PDFDocumentProxy.getStats` function, which in the default viewer is called for each rendered page, requires a round-trip to the worker-thread in order to obtain the current document stats. In the default viewer, we currently make one such API-call for *every rendered* page.
This patch proposes replacing that method with a *synchronous* `PDFDocumentProxy.stats` getter instead, combined with re-factoring the worker-thread code by adding a `DocStats`-class to track Stream/Font-types and *only send* them to the main-thread *the first time* that a type is encountered.

Note that in practice most PDF documents only use a fairly limited number of Stream/Font-types, which means that in longer documents most of the `PDFDocumentProxy.getStats`-calls will return the same data.[1]
This re-factoring will obviously benefit longer document the most[2], and could actually be seen as a regression for one-page documents, since in practice there'll usually be a couple of "DocStats" messages sent during the parsing of the first page. However, if the user zooms/rotates the document (which causes re-rendering), note that even a one-page document would start to benefit from these changes.

Another benefit of having the data available/cached in the API is that unless the document stats change during parsing, repeated `PDFDocumentProxy.stats`-calls will return *the same identical* object.
This is something that we can easily take advantage of in the default viewer, by now *only* reporting "documentStats" telemetry[3] when the data actually have changed rather than once per rendered page (again beneficial in longer documents).

---
[1] Furthermore, the maximium number of `StreamType`/`FontType` are `10` respectively `12`, which means that regardless of the complexity and page count in a PDF document there'll never be more than twenty-two "DocStats" messages sent; see 41ac3f0c07/src/shared/util.js (L206-L232)

[2] One example is the `pdf.pdf` document in the test-suite, where rendering all of its 1310 pages only result in a total of seven "DocStats" messages being sent from the worker-thread.

[3] Reporting telemetry, in Firefox, includes using `JSON.stringify` on the data and then sending an event to the `PdfStreamConverter.jsm`-code.
In that code the event is handled and `JSON.parse` is used to retrieve the data, and in the "documentStats"-case we'll then iterate through the data to avoid double-reporting telemetry; see https://searchfox.org/mozilla-central/rev/8f4c180b87e52f3345ef8a3432d6e54bd1eb18dc/toolkit/components/pdfjs/content/PdfStreamConverter.jsm#515-549

src/display/api.js

@@ -701,6 +701,16 @@ class PDFDocumentProxy {
           return this.fingerprints[0];
         },
       });
+
+      Object.defineProperty(this, "getStats", {
+        value: async () => {
+          deprecated(
+            "`PDFDocumentProxy.getStats`, " +
+              "please use the `PDFDocumentProxy.stats`-getter instead."
+          );
+          return this.stats || { streamTypes: {}, fontTypes: {} };
+        },
+      });
     }
   }
 
@@ -728,6 +738,24 @@ class PDFDocumentProxy {
     return this._pdfInfo.fingerprints;
   }
 
+  /**
+   * @typedef {Object} PDFDocumentStats
+   * @property {Object<string, boolean>} streamTypes - Used stream types in the
+   *   document (an item is set to true if specific stream ID was used in the
+   *   document).
+   * @property {Object<string, boolean>} fontTypes - Used font types in the
+   *   document (an item is set to true if specific font ID was used in the
+   *   document).
+   */
+
+  /**
+   * @type {PDFDocumentStats | null} The current statistics about document
+   *   structures, or `null` when no statistics exists.
+   */
+  get stats() {
+    return this._transport.stats;
+  }
+
   /**
    * @type {boolean} True if only XFA form.
    */
@@ -940,25 +968,6 @@ class PDFDocumentProxy {
     return this._transport.downloadInfoCapability.promise;
   }
 
-  /**
-   * @typedef {Object} PDFDocumentStats
-   * @property {Object<string, boolean>} streamTypes - Used stream types in the
-   *   document (an item is set to true if specific stream ID was used in the
-   *   document).
-   * @property {Object<string, boolean>} fontTypes - Used font types in the
-   *   document (an item is set to true if specific font ID was used in the
-   *   document).
-   */
-
-  /**
-   * @returns {Promise<PDFDocumentStats>} A promise this is resolved with
-   *   current statistics about document structures (see
-   *   {@link PDFDocumentStats}).
-   */
-  getStats() {
-    return this._transport.getStats();
-  }
-
   /**
    * Cleans up resources allocated by the document on both the main and worker
    * threads.
@@ -2392,6 +2401,8 @@ if (typeof PDFJSDev !== "undefined" && PDFJSDev.test("GENERIC")) {
  * @ignore
  */
 class WorkerTransport {
+  #docStats = null;
+
   constructor(messageHandler, loadingTask, networkStream, params) {
     this.messageHandler = messageHandler;
     this.loadingTask = loadingTask;
@@ -2433,6 +2444,10 @@ class WorkerTransport {
     return shadow(this, "annotationStorage", new AnnotationStorage());
   }
 
+  get stats() {
+    return this.#docStats;
+  }
+
   getRenderingIntent(
     intent,
     annotationMode = AnnotationMode.ENABLE,
@@ -2843,6 +2858,18 @@ class WorkerTransport {
       });
     });
 
+    messageHandler.on("DocStats", data => {
+      if (this.destroyed) {
+        return; // Ignore any pending requests if the worker was terminated.
+      }
+      // Ensure that a `PDFDocumentProxy.stats` call-site cannot accidentally
+      // modify this internal data.
+      this.#docStats = Object.freeze({
+        streamTypes: Object.freeze(data.streamTypes),
+        fontTypes: Object.freeze(data.fontTypes),
+      });
+    });
+
     messageHandler.on(
       "UnsupportedFeature",
       this._onUnsupportedFeature.bind(this)
@@ -3055,10 +3082,6 @@ class WorkerTransport {
     return this.messageHandler.sendWithPromise("GetMarkInfo", null);
   }
 
-  getStats() {
-    return this.messageHandler.sendWithPromise("GetStats", null);
-  }
-
   async startCleanup(keepLoadedFonts = false) {
     await this.messageHandler.sendWithPromise("Cleanup", null);