Toolkit: Multiline support for Toolkit Annotations

[svartalf]

This is a follow-up issue started at https://github.com/actions/toolkit/issues/186#issuecomment-546772099.

It is unclear from the recently added Annotation docs if ::debug/::warning/::error messages support multiline text in the {message} placeholder.

Why it is important? These Annotations can be used by various code linters and other similar tools, but their output easily could be spanned to multiple lines, for example:

pylint output:

W:108: Operator not followed by a space
     print >>sys.stderr, 'Unable to match %r', line
            ^

Rust compiler output:

warning: comparison is useless due to type limits
  --> example/src/lib.rs:20:8
   |
20 |     if pid < 0 {
   |        ^^^^^^^
   |
   = note: `#[warn(unused_comparisons)]` on by default

It would be nice to clarify if it is possible to use multiline strings over there and it is expected to be working correctly in the future.

[danechitoaie]

+1 At least some kind of initial feedback. Is it possible or not? And then maybe follow up with updating the docs.

[EricWF]

+1. This is one of the last blocking issues blocking me from moving to Github Actions. The output of my test suites failures are much longer than a single line, and I have no way to display them nicely.

[BrianHenryIE]

It can be achieved by using urlencoded newline %0A in place of \n.

I learned it from a comment by @PoisonousJohn at https://github.com/actions/starter-workflows/issues/68#issuecomment-581479448

I've confirmed it working for PHPUnit output and have a PR: https://github.com/mheap/phpunit-github-actions-printer/pull/14/files

And here's a screenshot of the outcome: https://i.imgur.com/nnSCPTG.png

Zero documentation anywhere, thank you @PoisonousJohn !

[EricWF]

I just discovered this, Cheers!
I can confirm it works and have used it successfully in a workflow.

PS. @BrianHenryIE Have you discovered a way to make problem matchers work with multi-line messages?