KTX2Loader: Improve parsing and transcoding process.

[donmccurdy]

Followup to #21094 and #21131.

Simplifies and improves KTX2Loader in several ways:

1. Internally uses a BasisTextureLoader instance for transcoding, reducing duplicate code.
2. Moves transcoding process into Web Workers (managed by BasisTextureLoader).
3. Imports parsing code from ktx-parse, simplifying the loader further.

The last item is not required, but is small (~1.5kb / 3kb, with and without tree-shaking) and should make it easier to add support for more features of the KTX format — which are not needed for glTF, and therefore not implemented yet — in the future.

After this change, the msc_basis_transcoder dependency is removed, and a warning is printed if the dependency's global variable is found. Users should update to use setTranscoderPath( ... ), the same as with BasisTextureLoader. This may need to be noted in the changelog.

[mrdoob]

Thanks!