Rewrote AudioDriverJavaScript to support multiple processor nodes.
The old (and deprecated) ScriptProcessorNode when threads are not
available, and the new AudioWorklet API when threads are enabled.
The new implementation uses two ring buffers and a shared state to
communicated with the AudioWorklet thread.
The audio.worklet.js JavaScript file is always added to the export
template, but only really used (and downloaded) in the thread build.
The API is implemented in javascript, and generates C functions that can
be called from godot.
This allows much cleaner code replacing all `EM_ASM` calls in our C++
code with plain C function calls.
This also gets rid of few hacks and comes with few optimizations (e.g.
custom cursor shapes should be much faster now).
This should fix some of the audio stuttering issues when the HTML5
export is compiled with threads support.
The API should be ported to AudioWorklet to (hopefully) be perfect.
That though, cannot be backported to 3.2 due to extra restriction of
AudioWorklet (which only runs in SecureContext, and needs a polyfill for
Safari).
