[3.2] [HTML5] Synchronous main, better persistence, handlers fixes, optional full screen.

[Faless]

3.2 version of #42178 :

This PR has most of the things in #41097 (excluding the virtual keyboard support):

• Refactor event handlers, moving add/remove logic to native/utils.js
• window event handlers no longer useCapture (fixes Engine locked "touchend" event in Web Exported game #33020)
• Canvas resize option now available on export (fixes Default HTML-5 export option forces scaling to fit viewport, even for non-resizable games #37205).

Plus, a rework of the initialization process to better handle file system synchronization (last commit, fixes #39643):

The engine now expects to emscripten FS to be setup and sync-ed before
main is called. This is exposed via Module["initFS"] which also allows
to setup multiple persistence paths (internal use only for now).

Additionally, FS syncing is done once for every loop if at least one
file in a persistent path was open for writing and closed, and if the FS
is not syncing already.

This should potentially fix issues reported by users where "autosave"
would not work on the web (never calling syncfs because of too many
writes).

[akien-mga]

Thanks!