[9.x] add dictionary to slug helper

[imdhemy]

The current implementation of the Str::slug() method already replaces the @ with at. The keyboard still has some other replaceable characters.

E.g 500$ bill becomes 500-dollar-bill instead of 500-bill.

Besides, some languages have different rules for the replacement of special characters.
With the current implementation, we can have something like:

أحمد-at-المدرسة

which should be:

أحمد-في-المدرسة

This PR adds a new parameter to the Str::slug() method to allow the developer to specify the replacement character for the special characters.

It does not break the current implementation, as the default value for the new parameter is ['@' => 'at'].

[milwad-dev]

is useful

[driesvints]

@milwad-dev please only comment when you have something to contribute to the discussion.

[shaedrich]

Wouldn't it be better to have a more diverse method signature like this:

Str::slug('500$ bill', [ '$' => fn($value) => /* do something */]);
Str::slug('500$ bill', fn($value) => /* do something */);
Str::slug('500$ bill', [ '/$|€/' => fn($value) => /* do something */]);

RegExp would even work quite well, since it is already used in that method.