[8.x] Add multibyte support to string padding helper functions

[clarkewing]

This PR adds support for multibyte strings to the Str::padBoth, Str::padLeft, and Str::padRight helper functions.

This is especially useful for localized strings, where accented characters or other multibyte glyphs were incorrectly padded before.

PHP has yet to implement a native mb_str_pad function so this PR adds a Str::mbStrPad helper function adapted from a comment on the PHP docs. There is an ongoing 19 year-old PHP request for this native function, but as it is yet to see any progress, it seems appropriate to implement a workaround in the framework.

[GrahamCampbell]

Laravel 8 is closed to new features. This PR adds a new non-private method.

[clarkewing]

Thanks for the speedy reply @GrahamCampbell! I made the Str::mbStrPad method protected. As it's my first time contributing to the framework, I'm open to any feedback.

[BrandonSurowiec]

It would probably be better to simply inline the change in every method instead of adding a protected helper. There are no "helper" methods for the other string methods that have multibyte support.

Only the second parameter is modified :

    public static function padRight($value, $length, $pad = ' ')
    {
        return str_pad($value, strlen($value) - mb_strlen($value) + $length, $pad, STR_PAD_RIGHT);
    }

I think this could be considered a bug fix and should target 8.x, but see what Taylor thinks.

[clarkewing]

Thanks for the helpful feedback @BrandonSurowiec! I committed a fix 😊

[BrandonSurowiec]

Looks good to me. I think it's ready to go 🔥