Add TypeScript for custom commands

[bahmutov]

No description provided.

[CypressCecelia]

I used these types in the RWA and they worked

/** Toast command from cypress-movie  */
   toast(message: string, options?: {}): Chainable<void>;

/**clearViewport command from cypress-movie */
    clearViewport(): Chainable<void>;

[CypressCecelia]

What would be a good way to include these? Is it something where we document the types or do we create an index.d.ts file that people can use or delete?