Threaded emscripten improvements

[BinBashBanana]

This cuts down on many of the proxied calls, as well as fixing the platform data that wasn't working when RetroArch was run off a pthread (see #17637). PROXY_TO_PTHREAD itself is removed, instead we create the pthread manually so that its stack size doesn't have to be identical to the browser thread stack size (good for the cores that need larger stacks).

emscriptenwebgl_ctx was made the default video context driver for all emscripten builds.

I've also completely redone the frontend for threaded RetroArch with a dark ozone-like theme, removed the jquery and bootstrap dependency, and added data migration from the old BrowserFS into the new OPFS.

Note that I've removed the userdata and bundle dirs for the threaded build as they are now redundant (everything is on the same mount), and removing them makes it more similar to the regular unix frontend.

I removed support for OPFS without PROXY_TO_PTHREAD; I couldn't get this to work? Supposedly it works with JSPI but that is currently unavailable.

Threaded RetroArch is still built with these makefile options: HAVE_WASMFS=1 PROXY_TO_PTHREAD=1

TODO (later): remove remaining proxied calls in rwebpad, write AudioWorklet audio driver

A few things need to happen before the threaded build replaces the old one:

• Bump emscripten on the buildbot to 4.0.4
• Special headers need to be set on web.libretro.com to enable shared memory
• CORS headers need to be added to buildbot.libretro.com
  • The bundle can be switched to the minimal bundle (pkg/emscripten/libretro-thread/libretro.worker.js) (the current one is quite large and takes 2-3 minutes to unzip into OPFS.)