x/tools/gopls: convert 'gofumpt' from formatter to code action provider

[stamblerre]

Add an analyzer that produces suggested fixes using https://pkg.go.dev/mvdan.cc/gofumpt/format?tab=doc.
The tricky part is that the dependency will have to live in the gopls module, since it's not a golang.org/x/... dependency. That means the analyzer will have to be in that module, and we won't be able to have any internal/lsp tests for this behavior until #35880 is resolved.

/cc @joshbaum @mvdan

[muirdm]

If people want to use gofumpt, wouldn't an option to use gofumpt for formatting be more useful than suggested fixes?

[stamblerre]

The suggested fixes would be categorized as "source.fixAll", which is a code action kind indicating that these are high-confidence fixes that can be applied on save. We already have similar analyzers and fixes for gofmt -s.

[muirdm]

I see, I knew I was missing something!

[gopherbot]

Change https://golang.org/cl/241985 mentions this issue: gopls, internal/lsp: support an extra formatting hook for gofumpt

[stamblerre]

Reopened this, as we've only added opt-in support for gofumpt as a formatter, not in the form of suggested fixes. I would strongly prefer a scenario in which gofumpt works more like staticcheck - it would make it more configurable for users and easier to adopt (users don't have to run it on save). To that end, I've filed mvdan/gofumpt#78 -- so this issue is blocked until that is resolved.