Upgrade `dmypy` to a language server (LSP)

Feature

I would like for dmypy to be a fully fledged language server. Obviously it would only support diagnostics (which would make this much simpler). But this would be very convenient from an editor point on view given the direction editor tooling is going.

Could probably use something like https://github.com/openlawlibrary/pygls to handle the low level details of LSP.

Pitch

Most editors and their plugins shell out and call mypy. But this can be quite slow on some projects since it needs to cold start every time (except for .mypy_cache).

Empieza leyendo la implementación de dmypy y la especificación de LSP; después, evalúa pygls como la biblioteca de bajo nivel propuesta. Define cómo debe exponer dmypy los diagnósticos mediante LSP e identifica las pruebas de editor o de protocolo necesarias para verificar el servidor. Se considera terminado cuando dmypy funciona como un servidor de lenguaje que proporciona diagnósticos a los editores.