Introduce RegexSyntaxValidator

[Al2Klimov]

Introduce RegexSyntaxValidator to validate the syntax of regular expressions, e.g., /PATTERN/[MODIFIERS].
The implementation uses preg_last_error_msg(), which is only available in PHP 8.0 or later, so the minimum PHP version requirement has been raised.

[sukhwinder33445]

The Modernize code PR has moved the Translation trait to the BaseValidator class and may be merged before this PR, so please remove the trait usage.

[lippserd]

Please rephrase the first commit as follows:

Require PHP 8.0 for `preg_last_error_msg()` to be available

This message indicates the reason at first glance and it's perfectly fine to have a maximum of 72 characters instead of 50 in the commit message.

Please add backticks to RegexSyntaxValidator in the second commit message and always do this when referring to code in commit messages and descriptions. This significantly improves readability in GitHub and also a little in non-GitHub contexts (at least, it doesn't impair readability in my opinion).

And please adjust the PR description to include the purpose of the validator, e.g.:

Introduce RegexSyntaxValidator to validate the syntax of regular expressions, e.g., /PATTERN/[MODIFIERS].
The implementation uses preg_last_error_msg(), which is only available in PHP 8 or later, so the minimum PHP version requirement has been raised.