Fix the default value of the $length parameter of grapheme_substr()

Store default parameter values of internal functions in arg info

Closes GH-5353. From now on, PHP will have reflection information
about default values of parameters of internal function

Store default parameter values of internal functions in arg info

Closes GH-5353. From now on, PHP will have reflection information
about default values of parameters of internal functions.

Co-authored-by: Nikita Popov <nikita.ppv@gmail.com>

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Do not include the same stub multiple times

Closes GH-5322