x/tools/gopls/internal/analysis/modernize: add modernizer for WaitGroup.Go

[adonovan]

The existing waitgroup analyzer reports misuses of wg.Add/Done. Now that WaitGroup.Go (#63796) is on course to be approved for go1.25, we should extend the analyzer to also report opportunities to use the new method instead of the old more complex code.

That is, it would report a diagnostic for:

wg.Add(1)
go func() { // "Go statement can be simplified using WaitGroup.Go"
        defer wg.Done()
         ...
}()

and offer a fix that changes it to:

wg.Go(func() {
         ...
})

The criteria should be:

• wg refers to a WaitGroup;
• a call to wg.Add directly precedes a go statement with a func literal.
• the sole use of wg beneath the go statement is a call to wg.Done, either deferred on the first line, or called directly on the last line.

However, modernize is only part of gopls, whereas waitgroup is (soon to be) part of the cmd/vet suite, so some care is required to ensure that only gopls gets the proposed new behavior.

[gopherbot]

Change https://go.dev/cl/661775 mentions this issue: gopls/internal/analysis/modernize: add modernizer for WaitGroup.Go