* This new audio stream allows to play multiple sounds and control them over time from code.
* It greatly simplifies tasks such as generative music (music generated from code) or audio.
This new type of stream was added with the goal of fixing audio blending in AnimationPlayer and AnimationTree, but can be used by others for their regular audio needs.
Does not fix anything currently, but should help implement #69758 properly.
Some demo code of how to use this:
```GDScript
var player = $SomeNode as AudioStreamPlayer
player.stream = AudioStreamPolyphonic.new()
var playback = player.get_stream_playback() as AudioStreamPlaybackPolyphonic
var id = playback.play_stream(preload("res://Clip1.ogg"))
await get_tree().create_timer(1).timeout
playback.set_stream_volume(id,-12) # Set volume to half after one second
await get_tree().create_timer(2).timeout
var id2 = playback.play_stream(preload("res://Clip2.ogg")) # 2 seconds later, start another clip
await get_tree().create_timer(1).timeout
playback.stop_stream(id) # 1 second later, kill the first clip
playback.set_stream_pitch_scale(id2,1.5) # Make the second clip go 50% faster
```
* From Godot 4.0 onwards, proper music looping is supported which requires setting the BPM and then the loop point in beats.
* As such, importing with loop by default does not serve much of a purpose. Its annoying to disable for SFX or clips that do not loop and it also requires manual intervention to loop music.
* This way, it should work out of the box for any sound effect or non looping audio clip, and manual work is required anyway for looping music.
The editor will use .NET 7 if it is installed and fall back to .NET 6 otherwise.
Exported projects will use .NET 7 or .NET 6 depending on the value of TargetFramework in the csproj.
This is using an adapted version of UAX#31 to not rely on the ICU
database (which isn't available in builds without TextServerAdvanced).
It allows most characters used in diverse scripts but not everything.
* Works for binary and text files.
* Makes EditorQuickOpen work with custom resources again.
* Information is cached and easily accessible.
Properly fixes#66179. Supersedes #66215 and supersedes #62417
**WARNING**: This required breaking backwards binary compatibility (.res and .scn files). Files saved after this PR is merged will no longer open in any earlier versions of Godot.
