Process a matched token.

@since 0.11.0 Split out from the `process()` method.

@param int    $stackPtr        The position of the current token in the stack.
@param string $group_name      The name of the group which was matched.
@param string $matched_content The token content (function name) which was matched.

@return int|void Integer stack pointer to skip forward or void to continue
normal file processing.