Expose a function to check whether a string is valid UTF-8

Description

Checking whether a string is valid UTF-8 is a common need, as many projects require UTF-8 encoding (to be able to use the u modifier in regex for instance, among other use cases).

As zend_string already has internal optimizations to remember that a string is valid UTF-8, implementing such function in core would provide several benefits over a userland implementation (it can both read the flag for a fast-path answer and set the flag at the end of the slow path to remember the validity for next usages of the string).

This could be implemented as a function is_valid_utf8(string $string): bool function

まず、PHP core 内の zend_string と、既存の UTF-8 の有効性の追跡または検証ロジックを見つけます。is_valid_utf8(string $string): bool 関数が公開され、後続の呼び出しに向けて有効性を確立しながら、記憶された有効性を使用できるようになれば、作業は完了です。