editor/getDiagnostics: advertise capability and handle server request

[ssjoleary]

Server can request LSP diagnostics from the editor via editor/getDiagnostics. eca-nvim currently does not advertise this capability during initialize and has no handler.

Work:

• Advertise editor/getDiagnostics in capabilities during initialize
• Add handler in mediator/server that calls vim.diagnostic.get() for the requested file(s)
• Map nvim diagnostic severity to ECA protocol severity levels
• Return results in expected format

Refs: upstream editor-code-assistant#26