Define a constant to easily add more characters to trim together with UTF8-spaces and line breaks

[utilmind]

The UTF8_SPACES constant allows me to easily add more characters to trim from the string.

E.g.
echo mb_trim(' —888-888-00-00-', UTF8_SPACES.'-–—');
will trim both spaces and dashes from the phone numbers.

[utilmind]

This PR not related to issue I posted separately (at https://github.com/PHP-Polyfills/mb_trim/issues).
This is different, but consider fixing that issue too, either by specifying the type of return value or removing the type hints.