[dev.typeparams] go/types: export the Config.GoVersion field

[dev.typeparams] go/types: export the Config.GoVersion field

Export the types.Config.GoVersion field, so that users can specify a
language compatibility version for go/types to enforce.

Updates #46648

Change-Id: I9e00122925faf0006cfb08c3f2d022619d5d54d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/334533
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>