Merge interfaces when merging TypeScript code

[miorel]

Some goodies all add methods to the same interface. See below example how map and filter both update Iterator<T> currently. We'd prefer to have a single declaration for Iterator<T> that combines map and filter together.

////////////////////////// BEGIN ADVENTURE PACK CODE ///////////////////////////
// Adventure Pack commit 9bf9e9a538edc5a3b379c903e854bbfae57247a9
// Running at: https://code-chronicles-code.github.io/leetcode-curriculum/

declare global {
  interface FunctionConstructor {
    returnThis<T>(this: T): T;
  }

  interface Iterator<T> {
    toIterable(this: Iterator<T>): IterableIterator<T>;
  }

  interface Iterator<T> {
    filter(
      this: Iterator<T>,
      callbackfn: (value: T, index: number) => unknown,
    ): Generator<T, void, void>;
  }

  interface Iterator<T> {
    map<TOut>(
      this: Iterator<T>,
      callbackFn: (element: T, index: number) => TOut,
    ): Generator<TOut, void, void>;
  }
}

Function.returnThis = function <T>(this: T): T {
  return this;
};

const iteratorPrototype = Object.getPrototypeOf(
  Object.getPrototypeOf([].values()),
) as Iterator<unknown, unknown, unknown>;

iteratorPrototype.toIterable = function <T>(
  this: Iterator<T>,
): IterableIterator<T> {
  (this as unknown as Record<symbol, unknown>)[Symbol.iterator] ??=
    Function.returnThis;
  return this as unknown as IterableIterator<T>;
};

iteratorPrototype.filter ??= function* <T>(
  this: Iterator<T>,
  callbackFn: (element: T, index: number) => unknown,
): Generator<T, void, void> {
  let index = 0;
  for (const element of this.toIterable()) {
    if (callbackFn(element, index)) {
      yield element;
    }
    ++index;
  }
};

iteratorPrototype.map ??= function* <TIn, TOut>(
  this: Iterator<TIn>,
  callbackFn: (element: TIn, index: number) => TOut,
): Generator<TOut, void, void> {
  let index = 0;
  for (const element of this.toIterable()) {
    yield callbackFn(element, index);
    ++index;
  }
};

/////////////////////////// END ADVENTURE PACK CODE ////////////////////////////