Try to expose more API-functionality in the TypeScript definitions

While these types apparently makes sense in TypeScript environments, we really don't want to extend the *public* API by simply exporting the relevant classes directly in `src/pdf.js` (since they should never be called/initialized manually).

Please see e.g. issue 12384 where this was first requested, and note that a possible work-around was also provided there. This patch simply implements that work-around[1], which will hopefully be helpful to TypeScript users.

---
[1] Based on the discussion in PR 13957, the two previous patches appear to be necessary for this to actually work.

src/pdf.js

@@ -14,6 +14,12 @@
  */
 /* eslint-disable sort-exports/sort-exports */
 
+// eslint-disable-next-line max-len
+/** @typedef {import("./display/api").PDFDocumentLoadingTask} PDFDocumentLoadingTask */
+/** @typedef {import("./display/api").PDFDocumentProxy} PDFDocumentProxy */
+/** @typedef {import("./display/api").PDFPageProxy} PDFPageProxy */
+/** @typedef {import("./display/api").RenderTask} RenderTask */
+
 import {
   addLinkAttributes,
   getFilenameFromUrl,