Make it possible to disable EmmyLua in server config

[hinell]

I'm more familiar with LDoc than EmmyLua. I never heard about this doc tags engine so I request to make it possible to disable it from the LSP config.

Thanks.

See also my reply over here: #392 (comment)

[hinell]

See related issue:

• Diagnostics: allow disabling EmmyLua luadoc annotations #2391