Show diagnostics even in files that don't run a server

[rchl]

Is your feature request related to a problem? Please describe.

Typescript server can report diagnostics for the tsconfig.json file but since it doesn't run on json files itself, the diagnostic is never visible (when opening the file).

Describe the solution you'd like

Diagnostics visible also in files that don't run a server.

[rchl]

Somewhat related, I've just noticed that rust-analyzer registers for didSave notification on files it doesn't support itself:

{
  "registrations": [
    {
      "id": "textDocument/didSave",
      "method": "textDocument/didSave",
      "registerOptions": {
        "documentSelector": [
          {
            "pattern": "**/*.rs"
          },
          {
            "pattern": "**/Cargo.toml"
          },
          {
            "pattern": "**/Cargo.lock"
          }
        ],
        "includeText": false
      }
    }
  ]
}

That said, I'm not sure why it's doing that since it also watches those using didChangeWatchedFiles so not sure what this adds.