Add ASTC support to BasisUniversal

[BlueCube3310]

Depends on #88464
Closes godotengine/godot-proposals#6357

Adds ASTC unpacking support to the BasisUniversal module.
This needs more testing on mobile devices, although having tried it out myself it seems to work correctly.

Small testing project:
BasisuASTC.zip

[fire]

The code review is good, but I don't have a setup for testing mobile at this current moment.

[Calinou]

Tested locally, I can only get it to show up as BPTC_RGBA on my Samsung Galaxy Z Fold4:

This is after enabling the Import ETC2/ASTC project setting and removing the .godot folder to force reimporting all resources. Both the editor and Android export template were recompiled.

[BlueCube3310]

That happens because bptc has a higher priority than astc, which means it gets chosen first if available. It looks like some mobile GPUs support bptc, so they will default to that instead.

[akien-mga]

Thanks!