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

Comienza localizando zend_string y la lógica existente de seguimiento o validación de la validez de UTF-8 en el PHP core. El trabajo estará terminado cuando se exponga una función is_valid_utf8(string $string): bool y pueda usar la validez recordada mientras la establece para las llamadas posteriores.