Add support for the basis compressed texture file extension

[JordyvanDortmont]

When converting a .gltf file to a .glb, the .basis compressed texture file extension is not accepted as a valid file header.

I added the file header, because soon basis will be supported in a GLTF (KhronosGroup/glTF#1612).

[lilleyse]

Based on the discussion in KhronosGroup/glTF#1612 it seems like the extension will require basis images to be contained in a KTX2 file (.ktx2 extension) not .basis.

Aside from that it would be good to add unit tests, one modifying gets image extension from buffer in getImageExtensionSpec.js and add two new ones in processGltfSpec.js mirroring the WebP tests. This will require adding a sample glTF and I think it's best to wait until the extension is finalized.

[JordyvanDortmont]

Thank you for the feedback. This was also confirmed here: mrdoob/three.js#17122 (comment).

I'll close the PR for now. Although there are, or were, quite some concerns around not directly including .basis files. We'll keep including them directly until the extension is finalized and if you'd like me to reopen and implement the unit tests and add a sample glTF, I'll gladly do so.