Update web export instructions for Godot 4.3+

[PgBiel]

Supersedes #56.

Instructions still need some testing, especially on Windows (let me know if anyone would like to help testing).

Most important changes are:

• Emscripten 3.1.62 or later is now recommended.
• Instructions to build with and without thread support have been added.
• We now mention the relation between Cross-Origin Isolation support for webservers and exporting with thread support.
• We now mention that Firefox is supported for extensions (albeit maybe not with all features, though I can't name a specific missing one).
• Other changes from updated instructions on exporting for web #56 were also inherited. In particular, the image with the export step in the editor now includes the Thread support checkbox.

[Bromeon]

This is an extremely valuable write-up, thanks so much! ❤️

Thanks also for the new screenshot, that's great.

I don't have much to add content-wise, comments are mostly formatting-related. Maybe eventually you can have 2 commits (1 with the previous author using co-authored-by, one using only your additions), and I can use merge instead of squash. If you find more than 2 commits are better for logical grouping, I can of course also keep more.

[PgBiel]

You're welcome! I'll also thank @illegalprime for the original modifications which served as inspiration.

I've applied some changes based on reviews (and also further ideas I had) and squashed the commits into two based on authorship, feel free to take a second pass and let me know of your thoughts.

[Bromeon]

Thank you very much for this great improvement! ❤️