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