Support for `workspace/diagnostic` from LSP 3.17

[versecafe]

Tracking issue for support on the LSP workspace/diagnostics request. Getting this working would be a major win over typescript-language-server and vtsls to get project wide diagnostics in editors where all issues are detected before each individual file is open the current reliance on didOpen makes editor workspace diagnostics less reliable.

[jakebailey]

I think this is something we want to do as a proper replacement to the (long experimental) typescript.tsserver.experimental.enableProjectDiagnostics setting, yes. It's certainly good that this is built-into the LSP as a request-based system, as opposed to us having to do this via push diagnostics (which we do not support outside providing diagnostics in tsconfig files).

[jakebailey]

I talked to Dirk about this and it turns out that this API is supposed to be used like old-school HTTP "long polling", which explains why the VS Code client just has a 2 second loop or something constantly asking for all diags. I don't really know how we'd test it, or what other constraints we want to put on it.