2.0.0

Split CI into one job per check for separate status reporting

Validate, lint, php-cs-fixer, PHPStan and PHPUnit each run as their
own job so each check appears as a distinct status on the run and on
PRs. Jobs run in parallel to keep total wall-time short.

A composite action at .github/actions/setup centralises the
PHP setup + composer install (with --ignore-platform-reqs and cache)
shared by the three jobs that need vendor/.

Release v1.0.0

Full Changelog: https://github.com/AngeGroup/Notifier/commits/v1.0.0