[Editor] Add a basic stamp editor (bug 1790255)

For now it allows to add a stamp annotation with an image selected from the file system.

web/app_options.js

@@ -103,6 +103,14 @@ const defaultOptions = {
     value: typeof PDFJSDev === "undefined" || !PDFJSDev.test("CHROME"),
     kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
   },
+  enableStampEditor: {
+    // We'll probably want to make some experiments before enabling this
+    // in Firefox release, but it has to be temporary.
+    // TODO: remove it when unnecessary.
+    /** @type {boolean} */
+    value: true,
+    kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
+  },
   externalLinkRel: {
     /** @type {string} */
     value: "noopener noreferrer nofollow",