Improve Regex return types + remove some of its options

Require PHP 8.4 and avoid PHP 8.5 deprecations

Project cleanup + fix bad class name

Upgrade dependencies + require PHP 8.1

First stable release

Add RemoveNonAlphabeticFilter

Bump dependency + improve attribute

- Use new squirrelphp/debug v2.x
- Make arguments for StringFilter attribute clearer
- Add PREG_UNMATCHED_AS_NULL to Regex::getMatches
- Add additional string filter attribute tests

Add regex wrapper

Documentation update + PHP 8.1 changes

- Add documentation for string testers
- Avoid strlen null deprecation message because a variable
  was not propery type checked
- Upgrade quality tools

Initialize model via reflection instead of new

Mandatory arguments lead to type errors so far because
the string filter extension tried to create a new class
via new instead of via reflection. Now with reflection
we ignore uninitialized properties of the class in our
string filter processor to be less error-prone.