BasisTextureLoader: Add deprecation warning

[donmccurdy]

This PR adds a deprecation warning to the BasisTextureLoader constructor, and removes the examples and documentation for the loader. Let's keep the loader around for a few releases to allow time for the transition. Instead, users should use Basis Universal compression in KTX2 (.ktx2) textures.

Note that Basis Universal is a compression codec. In the same way that Draco compression works best inside a full-featured model format like glTF or USD (rather than using its custom .drc files), Basis Universal should be used inside a full-featured texture format like KTX2.

Motivation:

• There is no advantage to using .basis instead of .ktx2, the compression is identical.
• KTX2 is more future-proof as a standard format, and has more features we can add over time, like HDR-compatible formats and color space information.
• Allows improvements to KTX2Loader in 2022, which I do not plan to backport to BasisTextureLoader.
• Reduce user confusion about the difference between the two, with users going down an error-prone path of trying to hack .basis textures into glTF files when .ktx2 is already supported.

[Mugen87]

It seems the CI error can be fixed if you remove the screenshot for webgl_loader_texture_basis.

[mrdoob]

Thanks!