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

Inizia individuando zend_string e la logica esistente di tracciamento o validazione della validità UTF-8 nel PHP core. Il lavoro è completato quando viene esposta una funzione is_valid_utf8(string $string): bool che può usare la validità memorizzata mentre la stabilisce per le chiamate successive.