x/tools/gopls: support textDocument/diagnostic

[findleyr]

gopls commands use a gopls/diagnoseFiles "non standard request" to implement file diagnostics for commands and testing. Unfortunately, this still publishes diagnostics asynchronously, and so suffers from syncronization problems (e.g. #59475).

There is now a textDocument/diagnostic LSP method that does exactly what we want: return diagnostics for a requested file. We should implement this method and use it instead.

Furthermore, we should remove support for non-standard requests. workspace/executeCommand is a better extensibility mechanism.

Implementing textDocument/diagnostic should be done with some care: if we advertise this capability, some clients may start requesting diagnostics rather than letting gopls compute them asynchronously. This may have lifecycle and performance implications.

CC @adonovan