feat: guard_msgs to escapes trailing newlines

[nomeata]

This makes trailing whitespace visible and protectes them against
trimming by the editor, by appending the symbol ⏎ to such a line (and
also to any line that ends with such a symbol, to avoid ambiguities in
the case the message already had that symbol).

(Only the code action output / docstring parsing is affected; the error message as sent
to the InfoView is unaffected.)

Fixes #3571

[leanprover-community-mathlib4-bot]

Mathlib CI status (docs):

• ❗ Std/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase f0a762ea4d2a78b1f0e484ffa9154c5f495d4c39 --onto ae492265fec103aa834d897bf9f68c94d10f0785. (2024-03-06 09:35:08)

[nomeata]

@kmill, would you be interested in having a look if this approach is reasonable?

[kmill]

I'm a bit uncertain about using a new symbol (⏎) for line breaks, but I've tried to come up with other solutions and there doesn't seem to be anything better.

1. ⏎ is unlikely to be used in messages, so we won't see very much of the ⏎⏎ encoding.
2. ⏎ is a good graphical representation for the concept.

Maybe @digama0 has thoughts about this? I approve conditional on Mario's approval (or if Mario conditionally approves on my approval).

[nomeata]

Thanks!

Fractional reviewing :-)

[digama0]

Yes, I'm fine with this solution. I don't think we will see much of this symbol anyway, because we want to avoid trailing whitespace altogether, but at least this won't cause heisenbugs that trigger only on save.