[11.x] slug() with alphanumeric separator

[faissaloux]

Fixes #54444

This PR made slug() fallback to default separator (-) when an alphanumeric separator been passed.

Examples

Before

str('nad')->slug('az'); // nazd

After

str('nad')->slug('az'); // nad

str('hello world')->slug('e'); // hello-word
str('hello world')->slug('2'); // hello-word

[shaedrich]

I don't like these silent fallbacks that might not be intuitive and therefore easily overlooked when something doesn't work as expected. Maybe, throwing an exception is more intuitive.