fix: remove incorrect version from composer

fix: preserve /u flag for regex patterns using Unicode property escapes

The __sanitizeCheckForJs method was unconditionally stripping the PCRE
/u modifier from regex patterns before passing them to JavaScript.
This broke patterns using Unicode property escapes (\p{...}), which
require the /u flag in JavaScript as well.

The fix now only strips /u when the pattern does not contain \p{...}
escapes. Also adds ext-mbstring as an explicit composer requirement.

references #193

Add support for min and max value validation

refactor: add getLastFieldset method to ValidForm

Also adds the ValidForm object to child elements using the “parent” meta.

feat: add support for “list items only” validation for select, radio …

…and checkboxes

refactor: support for version8

refactor: fix overly eager preparation for PHP 8.1

refactor: update PHP requirement to v7.3

Merge pull request #142 from WiebeCelery/master

Adds external validation.

Merge pull request #141 from flangfeldt/master

refactor: add override error logic to all cases in the Field Validator