removeSurrounding method
Removes from a String both the given prefix and suffix if and only
if it starts with the prefix and ends with the suffix.
Otherwise returns this String unchanged.
Implementation
String removeSurrounding({required String prefix, required String suffix}) {
  if (startsWith(prefix) && endsWith(suffix)) {
    return substring(prefix.length, length - suffix.length);
  } else {
    return this;
  }
}