Implement textDocument/rename and textDocument/prepareRename for LSP

[doriable]

This PR adds renaming capabilities to the Buf LSP, with
prepare rename request support.

[github-actions]

The latest Buf updates on your PR. Results from workflow Buf CI / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Dec 12, 2025, 6:25 PM

[doriable]

For valid symbol renames:

Clients will handle when textDocument/prepareRename returns an empty range:

[stefanvanburen]

my only question on this one: should we error when renaming a symbol to something that would conflict with another symbol in the scope?

e.g., if in gopls I attempt to rename lock to shutdown here (where the shutdown symbol already exists), it'll fail and log an error:

It seems like we'll happily rename to a conflicting name, and just get a lint error after:

[doriable]

my only question on this one: should we error when renaming a symbol to something that would conflict with another symbol in the scope?

Yeah, I think this is a good idea -- will put up a quick follow-up for this.