Add TypeScript type definitions

[neilzo]

We've duplicated a type definition for this repo a few times now:

• https://github.com/codecademy-engineering/Codecademy/blob/develop/webpack/assets/typings/ambients/40phaser.ts
• https://github.com/codecademy-engineering/static-sites/blob/main/src/typings/40phaser.d.ts
• https://github.com/codecademy-engineering/portal-app/pull/93/files#diff-54ebed486fc1809404352f16c8f88be80c16468e23170360c87bf0cc2404ea77

To DRY things up, we should add this definition to this repo.

The type def we've used (at time of this issue creation) is:

declare module '40phaser' {
  export type LaunchOptions = {
    phaser?: typeof Phaser;
    phaserOptions: {
      parent?: string;
    };
  };

  export const launch40Phaser: (options: LaunchOptions) => void;
}