go: Add `go-generate` runnables and tasks (#19987)

I was missing the `go generate` runnable from other editors so I figured
I'd implement one here! Now, comments of the form `//go:generate` can
prompt for the `go generate <package>` task. Meanwhile, I've also added
a global `go generate ./...` task.

~When making the global task, I noticed that the existing `go test
./...` task runs tests in subdirectories of the CWD of the active
editor, whereas I would really expect it to run all tests across my
project. I have changed to use the latter behavior (run relative to
project root) for both `go generate ./...` and `go test ./...`. Please
let me know if the prior behavior was intended, and I can revert.~

Release Notes:

- Added runnable and tasks for `go generate` commands