isTextOnly method
Checks if the entire string consists only of alphabet letters (a-z, A-Z).
@returns True if the string contains only letters and is not empty.
Example:
print('Flutter'.isTextOnly()); // true
print('Flutter Dev'.isTextOnly()); // false (contains space)
print('123'.isTextOnly()); // false
Implementation
bool isTextOnly() => RegExp(r'^[a-zA-Z]+$').hasMatch(this);