Add preg_last_error_msg() function

Provides the last PCRE error as a human-readable message, similar
to functionality existing in other extensions, such as
json_last_error_msg().

Add preg_last_error_msg() function

Provides the last PCRE error as a human-readable message, similar
to functionality existing in other extensions, such as
json_last_error_msg().

Closes GH-5185.

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