Generate a machine readeable list of builtins

[mochaaP]

Discussed in #71

^{Originally posted by traxys February 12, 2024}
As part of nixvim we generate Nix options for each of the available none-ls sources (see here).

This creates a bit of a mismatch between nixvim users and the none-ls docs, as not all sources are implemented, and we often have many PRs to add the sources that people need.
Would it be possible to generate a list of sources in something like JSON that we would be able to consume to automatically generate our options?

[mochaaP]

TODO: add schemas

[mochaaP]

/cc @traxys