go/doc/comment: add low-level parsing helpers

go/doc/comment: add low-level parsing helpers

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

Implement helpers to recognize old-style headings,
plain text (not marked up) URLs, and Go identifiers.

For #51082.

Change-Id: Ibabce72ef3ffd79a9d33366091f8c76ef27d0182
Reviewed-on: https://go-review.googlesource.com/c/go/+/397277
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>