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,
},

[raviqqe]

#4199 (comment)

Does anyone know if this is the editor's responsibility to make the diagnostic items unique, or servers should not mix them up? I couldn't find any mention in the LSP documentation...

• https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#diagnostic
• Chat GPT's claim: Do not mix them up

[barbedo]

I also can't find anything explicit in the specifications, which implies that both pushing and pulling should be supported independently by the client.

From the discussion in microsoft/language-server-protocol#1743, it seems that the original intent was to specify that the server pushing diagnostics is not needed if the client communicates that it supports pulling them, but that someone forgot to clarify this in the specs.

[justinmk]

Can improve it, meanwhile it sounds like people are using this so we can merge it.