Add WebPlugin to keep running in background

[cBournhonesque]

Fixes #144

All credit for this PR goes to @Nul-led who came up with the idea and did the entire implementation.
I just copy-pasted his code to put it in a WebPlugin. Thank you very much for this amazing solution!

In wasm, the main thread gets quickly throttled by the browser when it is hidden (e.g. when the user switches tabs).
This means that the app.update() function will not be called, because bevy's scheduler only runs app.update() when
the browser's requestAnimationFrame is called. (and that happens only when the tab is visible)

This is problematic because:

• we stop sending packets so the server disconnects the client because it doesn't receive keep-alives
• when the client reconnects, it also disconnects because it hasn't been receiving server packets
• the internal transport buffers can overflow because they are not being emptied

This solution spawns a WebWorker (a background thread) which is not throttled, and which runs
app.update() at a fixed interval. This way, the client can keep sending and receiving packets,
and updating the local World.

TODO:

• probably don't need world.clear_trackers()
• make interval configurable?
• confirm that the WebWorker doesn't get throttled even after long periods?

[simbleau]

This has universally great functionality - Should be a plugin one can easily cargo add webworker_update and add low-frequency updates to their application

[simbleau]

Magnifique

[Nul-led]

@simbleau thanks :p
ill probably make a crate for it then

[Nul-led]

re: configurable interval

1. we use format! to make it configurable at startup
2. we make an actual protocol for changing the duration (could be achieved by replacing setInterval with setTimeout)

[Nul-led]

re: trottled web workers
doesnt seem to happen (tested over 30 minutes), didnt confirm on webkit since i dont have a mac (feel free to check if anyone does), confirmed that it works for at least 7 minutes on android chromium based (more than that doesnt seem reasonable anyways)

[Nul-led]

Another note ill save here:
Lightyear doesnt need to send inputs while in background mode, so maybe worth looking into to save bandwidth?

[simbleau]

@simbleau thanks :p ill probably make a crate for it then

Please let me know when it's done, I'd add this to my bevy_rtc project today.

[Nul-led]

@simbleau thanks :p ill probably make a crate for it then

Please let me know when it's done, I'd add this to my bevy_rtc project today.

maybe make an impl urself for now then, ill prob wont have time until weekend