lib,lsp,none-ls: Add a standard way to handle automatic installation of packages that may not exist in nixpkgs

[traxys]

If such a package is used by an end user an error will be raised with a message of the form:

       error: Nixvim (plugins.none-ls.sources.formatting.textlint.package): No package is known for textlint, install externally and set this option to null

I'm open to changing that to a warning and returning null instead, though this would complicate the implementation as we would now need to check the priorities

[MattSturgeon]

I agree, this is looking nicer.

Thanks for continuing to work on this!

[GaetanLepage]

Looking very nice !

[MattSturgeon]

LGTM. Final nits and wording suggestions below:

[MattSturgeon]

LGTM, thanks!

[traxys]

@mergify queue

[mergify]

queue

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The pull request has been merged automatically at a2f0187

[mergify]

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