[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.

test/annotation_layer_builder_overrides.css

@@ -48,3 +48,13 @@
   margin: 0;
   padding: 0;
 }
+
+.annotationLayer .annotationTextContent {
+  position: absolute;
+  width: 100%;
+  height: 100%;
+  opacity: 0.4;
+  background-color: transparent;
+  color: red;
+  font-size: 10px;
+}

test/unit/annotation_spec.js

@@ -4101,6 +4101,35 @@ describe("annotation", function () {
         OPS.endAnnotation,
       ]);
     });
+
+    it("should extract the text from a FreeText annotation", async function () {
+      partialEvaluator.xref = new XRefMock();
+      const task = new WorkerTask("test FreeText text extraction");
+      const freetextAnnotation = (
+        await AnnotationFactory.printNewAnnotations(partialEvaluator, task, [
+          {
+            annotationType: AnnotationEditorType.FREETEXT,
+            rect: [12, 34, 56, 78],
+            rotation: 0,
+            fontSize: 10,
+            color: [0, 0, 0],
+            value: "Hello PDF.js\nWorld !",
+          },
+        ])
+      )[0];
+
+      await freetextAnnotation.extractTextContent(partialEvaluator, task, [
+        -Infinity,
+        -Infinity,
+        Infinity,
+        Infinity,
+      ]);
+
+      expect(freetextAnnotation.data.textContent).toEqual([
+        "Hello PDF.js",
+        "World !",
+      ]);
+    });
   });
 
   describe("InkAnnotation", function () {