Invoke linters, detect errors/warnings, show them as annotations. Supports PRs from forks, unlike other Actions.

Problem Matcher implementation in node.js

Shows xmllint errors in GitHub Actions as annotation with file and code line

Shows `php -l` errors in GitHub Actions as annotation with file and code line

Creates annotations for warnings and errors in gcc builds in GitHub Workflows

[WIP] Adds problem matchers for workflows invoking Angular CLI commands.