go/version: add new package

go/version: add new package

go/version provides basic comparison of Go versions,
for use when deciding whether certain language features
are allowed, and so on.

See the proposal issue #62039 for more details.

Fixes #62039

Change-Id: Ibdfd4fe15afe406c46da568cb31feb42ec30b530
Reviewed-on: https://go-review.googlesource.com/c/go/+/538895
Auto-Submit: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Bryan Mills <bcmills@google.com>