lastChars method
Get the last n graphemes (user-perceived characters) of a string.
Uses grapheme clusters for proper Unicode support, including emojis.
Returns the full string if its length is less than n.
Implementation
String lastChars(int n) {
// Handle invalid length.
if (n <= 0) return '';
final int charLength = characters.length;
// If requested length is greater than or equal to string length, return the whole string.
if (n >= charLength) return this;
// Return the last n characters.
return substringSafe(charLength - n);
}