Copy and modify mb_str_pad from str_pad

[youkidearitai]

I've mostly just copied from str_pad, so any pointers are welcome.

closes: #2701

[nielsdos]

Looks mostly good, thanks for working on this.
Just some remarks.

[nielsdos]

This whole paragraph may be replaced with the entity &mbstring.encoding.parameter; (the para tags should be removed too, i.e.: <listitem>&mbstring.encoding.parameter;</listitem>. (but I can't make a multi-line suggestion unfortunately)

[nielsdos]

And I believe the note about the ValueError exception should be in a separate errors section.
https://www.php.net/manual/en/function.mb-strlen.php puts it in the error section, although it seems outdated because it claims a warning is raised instead of a ValueError... (https://3v4l.org/E9s0i)

[nielsdos]

I filed an issue for the error conditions: #2751

[youkidearitai]

@nielsdos @TimWolla Thanks very much! I fixed.

[Girgias]

Is no ValueError emitted for negative lengths?

[youkidearitai]

Yes, no ValueError throws.

$ sapi/cli/php -r 'var_dump(mb_str_pad("あいうえお", -333, "🎉", STR_PAD_BOTH));'
string(15) "あいうえお"

[Girgias]

sad :(

[Girgias]

This also needs an entry in reference/mbstring/versions.xml to indicate it is available as of 8.3

[youkidearitai]

This also needs an entry in reference/mbstring/versions.xml to indicate it is available as of 8.3

@Girgias Thank you very much. Add to reference/mbstring/versions.xml .