Add helper method to do includes against reacted arrays

[stephenh]

Currently if you've got a list of reacted entities, and want to do an reactedEntities.includes(someOtherEntity), you basically always get a type error and have to as any around it.

  function asEntities<T extends Entity>(
    entities: ReadonlyArray<Omit<Reacted<T, any>, "fullNonReactiveAccess">>,
  ): readonly T[] {
    return entities as unknown as T[];
  }

  describe("types", () => {
    it("can do includes checks with entities", () => {
      // Given an array of reacted entities
      const books: Reacted<Book, "comments">[] = [];
      // And a regular book
      const b: Book = null!;
      // Then we can do includes checks
      expect(asEntities(books).includes(b)).toBe(false);
    });

    it("can do includes checks with other reactions", () => {
      // Given an array of reacted entities
      const books: Reacted<Book, "comments">[] = [];
      // And a different reacted book
      const b: Reacted<Book, {}> = null!;
      // Then we can do includes checks
      expect(asEntities(books).includes(b)).toBe(false);
      // And we can't do it with an author
      const a: Reacted<Author, {}> = null!;
      expect(asEntities(books).includes(a)).toBe(false);
    });
  });