x/tools/gopls: add StubMethods as a quick-fix for missing method diagnostics

[findleyr]

Reminder issue: right now StubMethods is available as a code-action when there are missing methods. For discoverability it should also be available as a quick-fix for the go/types error.

[findleyr]

Hmm, I think fixing this requires some deeper digging / refactoring of how type error analysis works. It seems like we have some error analyzers that show up as code lenses, and others that are supposed to be quick fixes that don't actually function as such:

E.g. if I have

var x intx := 1

We're supposed to suggest a quick fix changing the last statement to x = 1 (the nonewvars analyzer), but AFAICT this simply doesn't work.

Bumping out of the milestone, as this appears to be a larger project than expected.