go/doc/comment: parse and print code

go/doc/comment: parse and print code

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

Implement indented code blocks.

For #51082.

Change-Id: I0eacbf56e101424a875386cb6f26174b239561f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/397285
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>