Improve `:set-language` command

[A-Walrus]

Add text to completion for possible languages, and report error if user inputs invalid language.

[the-mikedavis]

Does this cause any trouble with the language server? Do we shut down the language server if we say :lang text?

[A-Walrus]

Does this cause any trouble with the language server? Do we shut down the language server if we say :lang text?

This seems to be an already existing bug. Currently on 22.05 when changing a language the language server is not killed. In fact if I change it to a language that I don't have the language server for (go in my case), not only is the language server not shut-down, but it is also still responding to requests: asciinema