[dev.cmdgo] cmd/go: add go mod initwork command

This command is used to create a go.work file with a set of modules
given in the arguments to the command.

For #45713
Change-Id: I09f8cefc5849dd43c234dc4a37091791fcc02ebe
Reviewed-on: https://go-review.googlesource.com/c/go/+/334936


Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>