Create `d.ts`

[jimmywarting]

Depends on #2

[eligrey]

I ended up using this type declaration for Conflux in Penumbra:

declare module '@transcend-io/conflux' {
  // TypeScript isn't aware of TransformStream
  /* eslint-disable no-undef */
  /** Conflux Zip Writer class */
  export class Writer extends TransformStream {
    /* eslint-enable no-undef */
    constructor();

    /** Write stream to zip */
    write(params: {
      name: string;
      lastModified: Date;
      stream(): ReadableStream;
    }): void;

    close(): void;
  }
}