[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/worker.js

@@ -536,7 +536,18 @@ class WorkerMessageHandler {
 
     handler.on("GetAnnotations", function ({ pageIndex, intent }) {
       return pdfManager.getPage(pageIndex).then(function (page) {
-        return page.getAnnotationsData(intent);
+        const task = new WorkerTask(`GetAnnotations: page ${pageIndex}`);
+        startWorkerTask(task);
+
+        return page.getAnnotationsData(handler, task, intent).then(
+          data => {
+            finishWorkerTask(task);
+            return data;
+          },
+          reason => {
+            finishWorkerTask(task);
+          }
+        );
       });
     });