cmd/api: require proposal # for new API features

Having the proposal numbers recorded in the API files
should help significantly when it comes time to audit
the new API additions at the end of each release cycle.

Change-Id: Id18e8cbdf892228a10ac17e4e21c7e17de5d4ff7
Reviewed-on: https://go-review.googlesource.com/c/go/+/392414


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>