x/tools/gopls: completion for `f.Fuzz` parameter

[hyangah]

Going one step further from #50198

It would be nice if gopls fills out the parameter automatically based on f.Add if it is present.

[gopherbot]

Change https://go.dev/cl/385974 mentions this issue: internal/lsp: Provide completions for test function definitions

[gopherbot]

Change https://go.dev/cl/387615 mentions this issue: internal/lsp: more precise completions for *testing.F fuzz methods

[findleyr]

Reopening, as I don't think this feature is quite working correctly.

If I am inside a fuzz function and type f.Fuz, the current implementation drops the completion to Fuzz. I think that's a bug -- it should only prune completions inside of a fuzz target.