[php-src] Issue #10853: Check UTF-8 validity for all constant strings on compile time

Issue: https://github.com/php/php-src/issues/10853 Author: mvorisek ### Description Currently the UTF-8 validity is checked on demand and cached to a string if not interned. However: - when a string is interned, such flag cannot be cached on runtime (at least not due TS) - it is not stored back to the source script cache/opcache, ie. the validity is checked on every request at least once - when a string is created from unvalidated source before checking, the validation cannot be cached - currently only "valid UTF-8" flag is cached , but when a string is "invalid UTF-8", nothing is cached at all This is a feature request to: a) check UTF-8 validity on compile time on every const string b) store a flag if UTF-8 validity was checked or not Thanks to #10436 the UTF-8 validity check is very fast and the compile time impact should be minimal.