1.6.7

• allow optional path objects for KeyPaths
  this eliminates the need to have non-nullable keys in sub-states that are addressed by KeyStateContext

  export type AppExampleState = {
      foobar: {
          example1: {
              app: { counter: number; text: string; },
          },
          example2?: {
              app: { counter: number; text: string; },
          },
          example3?: {
              app?: { counter: number; text: string; },
          },
      },
  };
  
  const state = createState<AppExampleState>({
      foobar: {
          example1: {
              app: {
                  counter: 0,
                  text: 'Hello, World!',
              },
          },
      }
  });
  
  const ctx1 = new KeyStateContext<typeof state, { counter: number; text: string; }>(state, "foobar.example1.app");
  const ctx2 = new KeyStateContext<typeof state, { counter: number; text: string; }>(state, "foobar.example2.app");
  const ctx3 = new KeyStateContext<typeof state, { counter: number; text: string; }>(state, "foobar.example3.app");

  in this example foobar.example1.app, foobar.example2.app and foobar.example3.app are the only valid keys