Check UTF-8 validity for all constant strings on compile time

Description

Currently the UTF-8 string validity is checked on demand and cached the 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.