isMongoId function
Checks if the string is a valid MongoDB ObjectId.
A MongoDB ObjectId is a 24-character hexadecimal string.
Example:
isMongoId('507f1f77bcf86cd799439011'); // true
isMongoId('507f1f77bcf86cd79943901'); // false (too short)
isMongoId('zzzf1f77bcf86cd799439011'); // false
Implementation
bool isMongoId(String str) => _isMongoId(str);