go/doc/comment: parse and print headings

go/doc/comment: parse and print headings

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

Implement both old-style and new-style headings, like:

	Text here.

	Old Style Heading

	More text here.

	# New Style Heading

	More text here.

For #51082.

Change-Id: I0d735782d0d345794fc2d4e1bdaa0251b8d4bba2
Reviewed-on: https://go-review.googlesource.com/c/go/+/397284
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>