isDigit function
Checks whether the given string is a digit from 0 to 9.
This function takes a String and returns true if the string represents a
digit from 0 to 9, and false otherwise.
Implementation
bool isDigit(final String char) {
const List<String> digits = [
'0',
'1',
'2',
'3',
'4',
'5',
'6',
'7',
'8',
'9',
];
return digits.contains(char);
}