Generate a machine readeable list of builtins

[mochaaP]

Discussed in https://github.com/nvimtools/none-ls.nvim/discussions/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