Propagate UTF-8 flag during Rope operations (#10915)

Imply UTF8 validity in implode function (#10780)

Sets the UTF-8 valid flag if all parts are valid, or numeric (which are valid UTF-8 by definition).

* remove unuseful comments

Imply UTF8 validity in implode function (#10780)

Sets the UTF-8 valid flag if all parts are valid, or numeric (which are valid UTF-8 by definition).

* remove unuseful comments

* Imply UTF8 validity in implode function

* revert zend_string_dup change

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Copy UTF-8 flag for str_repeat

Concatenating two valid UTF-8 strings produces a valid UTF-8 string

The UTF-8 valid flag needs to be copied upon interning,
otherwise strings that are concatenated at compile time lose t

Concatenating two valid UTF-8 strings produces a valid UTF-8 string

The UTF-8 valid flag needs to be copied upon interning,
otherwise strings that are concatenated at compile time lose this information.

However, if previously this string was interned without the flag it is not added
E.g. in the case the string is an existing class name.

Co-authored-by: Niels Dossche <7771979+nielsdos@users.noreply.github.com>

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Mark numeric strings as valid UTF-8

Add function in zend_test to check UTF8 flag is added

Also add test to check what strings are marked as having the flag