[2.1] Zend\Filter\Word\* no longer extends Zend\Filter\PregReplace

This PR fixes some my notes mentioned in #3376.

- `Zend\Filter\Word\*` no longer extends `Zend\Filter\PregReplace` but call `preg_replace` directly
- Set default value of `Zend\Filter\Word\AbstractSeparator->separator` to the same as defined in `__construct`
- `Zend\Filter\Word\SeparatorToSeparator::filter()` works without the need of the protected method `_separatorToSeparatorFilter()` and removed the protected method