none-ls: Add ability to specify custom servers

[alesauce]

Plugin: none-ls
Plugin repository: URL to the repository of the plugin
Is the plugin on nixpkgs: Yes

Extra information

Issue #97 was created in late 2022 with a request to add ability to specify custom servers for null-ls. It was closed due to null-ls being archived, but now that the none-ls fork has been created and is used in the Nixvim plugin ecosystem, I'd like to reopen the request to add the ability to specify custom servers.

Unrelated - thank you to everyone who has contributed on this project! Your hard work is very much appreciated!!

[garaiza-93]

If it's using a builtin without configuring it further, it shouldn't be impossible to implement. It's basically the name of the source and its functionality and inserting that into the list of sources.

What's concerning though is configuring each source. Looking at docs it's not entirely consistent. Note how some sources have specific options. But I think we could have more control by having an option for configuring the source.

What i imagine is that a type could be made that's an attribute set of options for the name and functionality of a source. Optionally, a configure option that takes in a raw Lua string.

But idk how I would take that data and transform it into Lua code :(

Here's something I came up with

  extraSourceOption = types.submodule {
    #what would we need to add a source?
    #name, functionality, optional configuration, package
    #user can add package themselves.
    options = {
      name = mkOption {
        type = types.str;
        description = "Name of source";
      };
      functionality = mkOption {
        type = types.enum [
          "code_actions"
          "completion"
          "diagnostics"
          "formatting"
          "hover"
        ];
        description = "Functionality of source";
      };
      withConfig = helpers.mkNullOrOption helpers.nixvimTypes.rawLua ''
        Configuration that will be inserted into with().
        See [none-ls docs](https://github.com/nvimtools/none-ls.nvim/blob/main/doc/BUILTINS.md)
      '';
    };
  };

[garaiza-93]

Actually, looking at the helpers for none-ls, a lot of the heavy lifting has already been done.