backspace method
Deletes the character before the cursor (backspace).
Returns true if a character was deleted.
Implementation
bool backspace() {
if (_cursorPosition == 0) return false;
final before = text.substring(0, _cursorPosition - 1);
final after = text.substring(_cursorPosition);
_buffer.clear();
_buffer.write(before);
_buffer.write(after);
_cursorPosition--;
return true;
}