isDigitsOnly method
Checks if the entire string consists only of numerical digits (0-9).
@returns True if the string contains one or more digits and no other characters (including spaces).
Example:
print('12345'.isDigitsOnly()); // true
print('123 a'.isDigitsOnly()); // false
Implementation
bool isDigitsOnly() => RegExp(r'^\d+$').hasMatch(this);