Scan for a single complete and valid GVariant type string in string. The memory pointed to by limit (or bytes beyond it) is never accessed.
If a valid type string is found, endptr is updated to point to the first character past the end of the string that was found and TRUE is returned.
If there is no valid type string starting at string, or if the type string does not end before limit then FALSE is returned.
For the simple case of checking if a string is a valid type string, see Variant.typeStringIsValid.