go/doc/comment: parse and print lists

go/doc/comment: parse and print lists

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

Implement lists, like:

	Three numbers:

	  - One
	  - Two
	  - Three

For #51082.

Change-Id: Id87d9c19bca677be968f3803809a9ea6c705f3ad
Reviewed-on: https://go-review.googlesource.com/c/go/+/397286
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>