feat: rumdl

[raviqqe]

This PR adds support for the rumdl language server for Markdown files.

As its MDX support is questionable, the configuration is applied to only pure Markdown files.

[barbedo]

I'm getting duplicated diagnostics when using this configuration. It seems that rumdl supports both publishDiagnostics and diagnostics.

I don't know if there is a way to tell rumdl to avoid pushing diagnostics, but I silenced it by stubbing the publishDiagnostics handler:

handlers = {
  ["textDocument/publishDiagnostics"] = function() end,
},