Improve capability handling

[jose-elias-alvarez]

Neovim currently doesn't have support for dynamic capability registration, though there is an open PR. Supporting it will let the null-ls server start up with no capabilities, then register its actual capabilities based on a user's registered sources.

In the meantime, I want to improve the way null-ls handles requests to avoid cases like this one, where null-ls is listed as a formatting source even though it can't provide formatting for a filetype (diagnostics and code actions are fine, since an empty response doesn't do anything negative). My tentative idea is to override client.supports_method, which I think should let us catch this earlier, but I need to look into this a bit more when I have time.

[jose-elias-alvarez]

Should be closed by 8b30746.