[HTML5] Refactor audio drivers. Implement AudioWorklet w/o threads.

Performances are not great in general, bad on Firefox, on Chrome, well,
it could be an improvement. Leave it as a fallback for now, but can be
forced via project settings if desired (or custom JavaScript logic via
the "args" option).

I'm actually surprised this works, it involves so many allocations, but
there's no way around it when SharedArrayBuffer is not available :(.

platform/javascript/os_javascript.h

@@ -40,7 +40,7 @@
 
 class OS_JavaScript : public OS_Unix {
 	MainLoop *main_loop = nullptr;
-	AudioDriverJavaScript *audio_driver_javascript = nullptr;
+	List<AudioDriverJavaScript *> audio_drivers;
 
 	bool idb_is_syncing = false;
 	bool idb_available = false;