Add a bindings field for implementations in other languages

[Zethson]

Summary

• Adds a bindings field to the schema: a list of {language, name, url} for implementations of a package's objects or on-disk format in other languages, restricted to ones maintained under the scverse org.
• Populates it for AnnData (anndataR, anndata-rs) and MuData (Muon.jl).
• The language enum moves to $defs so the package's own language and a binding's language stay in sync.
• Binding URLs get link-checked like project_home and the rest.
• Prerequisite for Show and filter by the languages a package is available in scverse.github.io#316, which currently hardcodes the same links in the site's front matter, and for the language filter @flying-sheep asked for there.

Ran validate-registry locally: all three URLs validate, packages.json carries the field, pre-commit passes.

[Zethson]

Ah whops, I had wanted to think about and iterate on this before merging but if both of you like this version already then uhm let's ship it haha.

Thanks

[flying-sheep]

Haha I do. I guess I’d have expected the “draft PR” mode if it was to be iterated on.