[13.x] Fix multibyte case-insensitive matching in Str::replace and Str::remove

[marekmiklusek]

Str::replace() and Str::remove() with $caseSensitive = false delegate
to str_ireplace(), which only case-folds ASCII characters. Multibyte
characters are never matched case-insensitively:

Str::replace('ž', 'X', 'Žltý kôň', false);     // "Žltý kôň" — nothing replaced
Str::replace('KÔŇ', 'pes', 'žltý kôň', false); // "žltý kôň" — nothing replaced
Str::remove('ž', 'Žltý', false);               // "Žltý" — nothing removed

This is inconsistent within the class itself -> Str::contains() considers
the same strings a case-insensitive match:

Str::contains('Žltý', 'ž', ignoreCase: true); // true

ASCII-only search terms keep using str_ireplace() exactly as before.
Multibyte terms are matched via preg_quote + /iu patterns with a callback
replacement, replicating str_ireplace() array pairing and sequential
semantics. Subjects that are not valid UTF-8 fall back to str_ireplace(),
preserving the previous behavior.

[shaedrich]

I'm not sure but this probably breaks on non-array iterables, right?

[marekmiklusek]

Good catch on the docblock! The method itself can only receive normalized
input, both public entry points (replace() and remove()) convert
Traversable searches via iterator_to_array() before calling it, and the
existing collect() test in testReplace covers that path. But the
iterable<string> docblock on the helper was inaccurate for what it
actually accepts, updated it to array|string. Thanks Man!

[shaedrich]

You're welcome!

[shaedrich]

You could probably narrow this down to

Suggested change

	* @param array|string $search
	* @param array|string $replace
	* @param array|string $subject
	* @param string|string[] $search
	* @param string|string[] $replace
	* @param string|string[] $subject