Add emmylua as second option for Lua LS

[xuhuanzy]

Is your feature request related to a problem? Please describe.

Provide an LSP switching configuration for lua, but I'm not sure how the author views the scenario of having multiple LSPs for one language.

For lua, the most widely used LSP currently is luals, which is also the default LSP downloaded and activated. luals is derived from emmylua.
However, we have completely rewritten emmylua using rust, offering more features (some of which cannot be achieved with luals), and it is the most widely used LSP in IntelliJ IDEA (thanks to the user base of the original emmylua project).

Describe the solution you'd like
A clear and concise description of what you want to happen.

The author proposes a specification to handle situations where multiple LSPs exist for the same language.

[xuhuanzy]

Although the main authors of the three projects, emmylua, luals, and emmylua-rust, have reached a consensus on compatibility at a fundamental level, it is regrettable that they are not fully compatible. Even worse, both of them have surpassed one million downloads.

[MischaPanch]

It's not a problem to have multiple LS implementations for one language, we already have that for python, C# and ruby.

One implementation should be the main one (inferred automatically), the others should be marked as experimental. An experimental LS can be used by adjusting the language entry in project.yml.

If you want to add a PR with a second LS implementation for lua, we'd be happy to review it

[xuhuanzy]

Can you provide detailed examples? How to switch? language: lua-emmylua?

[MischaPanch]

It's handled in the module ls_config. There we have

    def is_experimental(self) -> bool:
        """
        Check if the language server is experimental or deprecated.
        """
        return self in {self.TYPESCRIPT_VTS, self.PYTHON_JEDI, self.CSHARP_OMNISHARP, self.RUBY_SOLARGRAPH, self.MARKDOWN}

When a language server is experimental, it's not inferred automatically at project creation. You can use for example solargraph instead of the default ruby ls by specifying language: ruby_solargraph in the project.yml.

[MischaPanch]

If it wasn't clear - each language server needs to be implemented. So if you want emmylua, you'd have to implement it (following the guide we mention in our readme) and register it as experimental.