CLI to retrieve errors from the current build

[cwong-ocaml]

This introduces dune diagnostics, which can be used as a lightweight alternative to spinning up a separate RPC client

[rgrinberg]

I don't see where this command can be useful, but the implementation looks right.

[cwong-ocaml]

There are two use cases I have in mind:

• It's a bit more lightweight than writing an entire separate application using the client library. For example, in vim, I'd rather shell out to dune diagnostics easily to populate the error list than set up a persistent client connection
• When debugging strange editor issues, it can be useful to see if the client's state matches the server's internal state, and this is a relatively direct way of doing so