Revisit support for multiline messages in problem matchers

[tido64]

Due to a limitation in the format of problem matchers, it's currently not possible to extract multiline messages. For input such as:

TestTestApp#test_resources_pod_returns_spec_path [test/test_test_app.rb:31]:
Expected: ".."
  Actual: "."

"Expected: …" and "Actual: …" are two separate errors with this matcher. We are currently not able to match output such as:

TestTestApp#test_resources_pod_writes_podspec [test/test_test_app.rb:50]:
--- expected
+++ actual
@@ -1 +1 @@
-["dist/assetd", "dist/main.jsbundle"]
+["dist/assets", "dist/main.jsbundle"]

This issue has been reported in actions/toolkit#193 and actions/toolkit#319.

Originally posted by @tido64 in #61