Idea: a way to "lint" server messages to see if they're respecting client capabilities

Clients provide servers with lots of information about what features they support. But it's very easy for the server to just ignore this and send the client things that they may not understand.

Now, I _think_ this is usually fine because usually this just means the server sends extra fields, which the client will ignore. But in some cases it can be important: for example if you send `documentChanges` in `WorkspaceEdit` to a client that only supports `changes`, then it may do nothing.

What I was thinking is that we could write, for each client capability, a "linter" that checks certain messages to see if they contain fields that the client hasn't stated that they support. Then we could, e.g. emit warnings if we see such fields, helping server authors notice if they're not honouring client capabilities.