[dev.typeparams] cmd/gofmt: add the -G flag to allow generic code

Add support for type parameters to cmd/gofmt, gated behind the -G flag.

The test was based on a test from go/printer, slightly modified to
exercise more formatting.

Change-Id: I489bcb3ad06e1ed4e6d9f5bc79825e60dcfe9953
Reviewed-on: https://go-review.googlesource.com/c/go/+/291011


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>