go/doc/comment: add paragraph parsing and test framework
[This CL is part of a sequence implementing the proposal #51082.
The design doc is at https://go.dev/s/godocfmt-design.]
Implement parsing of plain text doc paragraphs,
as well as a txtar-based test framework. Subsequent CLs will
implement the rest of the possible markup.
For #51082.
Change-Id: I449aac69b44089f241fde8050ac134e17cb25116
Reviewed-on: https://go-review.googlesource.com/c/go/+/397278
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>