[Annotation] Add a div containing the text of a FreeText annotation (bug 1780375)

An annotation doesn't have to be in the text flow, hence it's likely a bad idea
to insert its text in the text layer. But the text must be visible from a screen
reader point of view so it must somewhere in the DOM.
So with this patch, the text from a FreeText annotation is extracted and added in
a div in its HTML counterpart, and with the patch #15237 the text should be visible
and positioned relatively to the text flow.

src/core/annotation.js

@@ -941,6 +941,57 @@ class Annotation {
     return null;
   }
 
+  get hasTextContent() {
+    return false;
+  }
+
+  async extractTextContent(evaluator, task, viewBox) {
+    if (!this.appearance) {
+      return;
+    }
+
+    const resources = await this.loadResources(
+      ["ExtGState", "Font", "Properties", "XObject"],
+      this.appearance
+    );
+
+    const text = [];
+    const buffer = [];
+    const sink = {
+      desiredSize: Math.Infinity,
+      ready: true,
+
+      enqueue(chunk, size) {
+        for (const item of chunk.items) {
+          buffer.push(item.str);
+          if (item.hasEOL) {
+            text.push(buffer.join(""));
+            buffer.length = 0;
+          }
+        }
+      },
+    };
+
+    await evaluator.getTextContent({
+      stream: this.appearance,
+      task,
+      resources,
+      includeMarkedContent: true,
+      combineTextItems: true,
+      sink,
+      viewBox,
+    });
+    this.reset();
+
+    if (buffer.length) {
+      text.push(buffer.join(""));
+    }
+
+    if (text.length > 0) {
+      this.data.textContent = text;
+    }
+  }
+
   /**
    * Get field data for usage in JS sandbox.
    *
@@ -3250,6 +3301,10 @@ class FreeTextAnnotation extends MarkupAnnotation {
     this.data.annotationType = AnnotationType.FREETEXT;
   }
 
+  get hasTextContent() {
+    return !!this.appearance;
+  }
+
   static createNewDict(annotation, xref, { apRef, ap }) {
     const { color, fontSize, rect, rotation, user, value } = annotation;
     const freetext = new Dict(xref);