Expose worker.d.ts

[exKAZUu]

worker.d.ts is required when using a custom worker, so I think we should expose worker.d.ts (via index.d.ts ?).

Note marp worker on CDN is incompatible with the latest marp-react (i.e. master branch) so we must use a custom worker when using the latest marp-react.

[exKAZUu]

May I change https://github.com/marp-team/marp-react/blob/master/src/worker.ts#L6 to export function initializeWorker or something? Are there some reasons where we use default export?
Because named export can easily expose this function via index.d.ts.

[yhatt]

I recommend to take a way with no depending the structure of export type: emit type definition of worker.d.ts into the same directory as lib/worker.js rather than types directory.