XFA - Move the fake HTML representation of XFA from the worker to the main thread

- the only goal of this patch is to be able to get synchronously the fake html when printing from firefox:
    - in order to print we need to inject some html in beforeprint callback but we cannot block in waiting for all the pages.
  - from a memory point of view: it doesn't change anything since the fake HTML is deleted in the worker;
  - this way we don't break any assumptions.

src/display/api.js

@@ -695,7 +695,15 @@ class PDFDocumentProxy {
    * @type {boolean} True if only XFA form.
    */
   get isPureXfa() {
-    return this._pdfInfo.isPureXfa;
+    return !!this._transport._htmlForXfa;
+  }
+
+  /**
+   * @type {Object | null} An object representing a HTML tree structure
+   * to render the XFA, or `null` when no XFA form exists.
+   */
+  get allXfaHtml() {
+    return this._transport._htmlForXfa;
   }
 
   /**
@@ -1255,8 +1263,8 @@ class PDFPageProxy {
    *   are {Object} with a name, attributes (class, style, ...), value and
    *   children, very similar to a HTML DOM tree), or `null` if no XFA exists.
    */
-  getXfa() {
-    return (this._xfaPromise ||= this._transport.getPageXfa(this._pageIndex));
+  async getXfa() {
+    return this._transport._htmlForXfa?.children[this._pageIndex] || null;
   }
 
   /**
@@ -1552,7 +1560,6 @@ class PDFPageProxy {
     this.objs.clear();
     this._annotationsPromise = null;
     this._jsActionsPromise = null;
-    this._xfaPromise = null;
     this._structTreePromise = null;
     this.pendingCleanup = false;
     return Promise.all(waitOn);
@@ -1588,7 +1595,6 @@ class PDFPageProxy {
     this.objs.clear();
     this._annotationsPromise = null;
     this._jsActionsPromise = null;
-    this._xfaPromise = null;
     this._structTreePromise = null;
     if (resetStats && this._stats) {
       this._stats = new StatTimer();
@@ -2456,6 +2462,8 @@ class WorkerTransport {
 
     messageHandler.on("GetDoc", ({ pdfInfo }) => {
       this._numPages = pdfInfo.numPages;
+      this._htmlForXfa = pdfInfo.htmlForXfa;
+      delete pdfInfo.htmlForXfa;
       loadingTask._capability.resolve(new PDFDocumentProxy(pdfInfo, this));
     });
 
@@ -2812,12 +2820,6 @@ class WorkerTransport {
     });
   }
 
-  getPageXfa(pageIndex) {
-    return this.messageHandler.sendWithPromise("GetPageXfa", {
-      pageIndex,
-    });
-  }
-
   getStructTree(pageIndex) {
     return this.messageHandler.sendWithPromise("GetStructTree", {
       pageIndex,