Add regexify formatter

Transforms a basic regular expression into a random string satisfying the expression.

```php
$faker->regexify('[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,4}'); // sm0@y8k96a.ej
```

Regex delimiters '/.../' and begin/end markers '^...$' are ignored.

Only supports a small subset of the regex syntax. For instance, unicode, negated classes, unbouned ranges, subpatterns, back references, assertions, recursive patterns, and comments are not supported. Escaping support is extremely fragile. See [icomefromthenet/ReverseRegex](https://github.com/icomefromthenet/ReverseRegex) for a more robust implementation.

This method is also VERY slow. Use it only when no other formatter can generate the fake data you want. For instance, prefer calling `$faker->email` rather than `regexify` with the previous regular expression.

Also note than `bothify` can probably do most of what this method does, but much faster. For instance, for a dummy email generation, try `$faker->bothify('?????????@???.???')`.