BasisTextureLoader: Refactor

[donmccurdy]

Provides a parseInternalAsync method that KTX2Loader can begin using in a future PR, so that the transcoding logic can be mostly shared between the two loaders. This will allow us to remove the msc_basis_transcoder files and have only a single Basis transcoder in the project. The change also improves selection of the target format into which Basis textures are transcoded.

Tested on:

• ✅ iPhone 11, Safari
• ✅ iPhone SE, Safari
• ✅ iPad, Safari
• ✅ macOS (Safari, Firefox, Chrome)
• ✅ Pixel 4 (Chrome, Firefox)
• ✅ Nexus 5X (Chrome)

Some parts of this change require using the latest version of the Basis transcoder, checked in with #21094. This should be noted as a "breaking change" in the release notes, since users may have copied the old transcoder files into a local directory and would need to update them now.

[mrdoob]

Thanks!

[donmccurdy]

@Mugen87, I'm having trouble updating running modularize.js on the dev branch (see #21138) and can't address your comments at the moment.

[mrdoob]

Should be working again #21138 (comment)

[donmccurdy]

/cc #21144