Allow adding additional functions to `_worker.js`

Describe the problem

Cloudflare Queues is currently in open beta. It would be nice to be able to consume messages from the same worker that handles HTTP requests, as opposed to having to creating a separate worker. For a worker to consume from a queue a new queue function needs to be defined on the worker object.

// worker.js
export default {
  async fetch(request, env, ctx) {
    // Handle request...
  },
  async queue(batch, env) {
    // Process batch...
  },
};

Currently there is no easy way to add this queue function to the woker object in the generated _worker.js.

Other Cloudflare services such as Email Workers follow a similar approach.

Describe the proposed solution

I'd suggest a special src/cloudflare-worker.js file which would export functions such as queue which then during compilation get added to the _worker.js file.

// src/cloudflare-worker.js

export const queue = (batch, env) =>  // Process batch...

Alternatives considered

No response

Importance

would make my life easier

Additional Information

No response

Start by locating how the generated _worker.js is produced and review the proposed src/cloudflare-worker.js entry point. Check the closed pull requests linked to this issue for prior implementation context. Done means additional worker functions such as queue are available in the generated worker alongside HTTP handling.