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

首先定位 PHP core 中的 zend_string 以及现有的 UTF-8 有效性跟踪或验证逻辑。当公开一个 is_valid_utf8(string $string): bool 函数，并且该函数能够使用已记住的有效性，同时为后续调用建立有效性时，工作就完成了。