Upgrade Mozilla PDF.js library

[edeleastar]

Currently the PDF component used in talk learning object:

• PDF.js
• Talk.svelte

... is pined at version 3.x:

    "pdfjs-dist": "3.11.174",

Version 4.x is currently not functioning with Svelte/SvelreKit due to some javascript issues. Revise the Talk object to be able to use 4.x of pdf-dist.

[lgriffin]

Fix pushed for this, required some refactoring and handling of the async loading capability but this is now resolved and working.

[edeleastar]

Closed and implemented with thanks @lgriffin