Prevent HasSlug::getUtf8Slug() from guessing encoding by default

[zachgarwood]

Description

By default, the method was guessing from a list of all possible encodings what the encoding of the given string was. This change allows the method caller to supply the list of possible encodings. If no encodings are provided, it will use the default_charset setting. See https://www.php.net/manual/en/function.mb-convert-encoding.php.

This is an extremely narrow edge case, but it is possible to encounter an encoding clash, where the given string just so happens to be valid in two different encodings. We currently only have one example, but we have encountered this issue a couple other times in the past, but did not record them.

The string in question is "Tusk". Below, we see that the string was converted to "畔歳" because mb_convert_encoding() guessed it was encoded in UCS-2LE, an obsolete encoding that was superceded by UTF-16:

vagrant@homestead:~/website$ php artisan tinker
Psy Shell v0.12.8 (PHP 8.3.23 — cli) by Justin Hileman
> $str = mb_convert_encoding((string)'Tusk', 'UTF-8', mb_list_encodings());
= "畔歳"
> mb_detect_encoding('Tusk', mb_list_encodings());
= "UCS-2LE"

Under the hood, mb_convert_encoding() uses mb_detect_encoding() to guess the encoding, and from the documentation, we can see that this isn't always reliable:

https://www.php.net/manual/en/function.mb-detect-encoding.php