go/doc/comment: parse and print identifiers, automatic links

go/doc/comment: parse and print identifiers, automatic links

[This CL is part of a sequence implementing the proposal #51082.
The design doc is at https://go.dev/s/godocfmt-design.]

Implement parsing and printing of unmarked identifiers
and automatic URL links in plain text.

For #51082.

Change-Id: Ib83ad482937501a6fc14fa788eab289533a68e3a
Reviewed-on: https://go-review.googlesource.com/c/go/+/397280
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>