[rcore][RGFW] Add new backend option: PLATFORM_WEB_RGFW

[ColleagueRiley]

This is a WIP pull request.

[ColleagueRiley]

Issues thus far:

• DND file drops are called "???"

• Fullscreen toggle causing screen to go black

• joystick input example not doing anything

• Enter key not being registered

• more testing has to be done

(This seems to be a problem with both backends, but it might just be an Emscripten thing)

• Canvas window sizing creating a viewport (also causes mouse X & Y to mess up)

[raysan5]

@ColleagueRiley Thank you very much for working on this new platform backend! Exiting to have a GLFW alternative for PLATFORM_WEB! Keep up the great work!