Consider removal of msc_* js/wasm standalone transcoder

[donmccurdy]

With the latest updates to the https://github.com/BinomialLLC/basis_universal/ repository, it is possible to decode KTX files (not just .basis files) with that transcoder. The msc_* standalone transcoder existed to fill that gap, and mostly represented a mirror of code from that repository otherwise. So I think we should remove the transcoder now, or at least omit it from the next release and from documentation. If we want to keep the build targets around a bit longer for any users who need to migrate (i.e. are depending on the feature in the beta release) that is fine too. Is this OK?

[donmccurdy]

@bghgary the three.js implementation, updated from the msc_* transcoders to the binomial transcoder, depends on KTX-Parse and may be found here:

• https://github.com/mrdoob/three.js/blob/dev/examples/jsm/loaders/KTX2Loader.js
• https://github.com/mrdoob/three.js/blob/26aec21a9672b9eab6592e42fadeb732ca996534/examples/jsm/loaders/BasisTextureLoader.js#L451-L463

Unfortunately it's a bit roundabout to support different inputs, since we are trying to support both .basis and .ktx2 for now.