Add Rector

[Nielsvanpach]

Added Rector as dev-dependency

• Run as composer rector for dry-runs
• Run as ./vendor/bin/rector to execute

Updates

• Run with PHP 8.1 ruleset
• Remove deadcode
• Run with code styling ruleset
• Added type declarations where missing
• Run workflows on all branches. This allows contributors to run on their forks.

[Nielsvanpach]

All ready