🐛 BUG: Class instances cannot be used to create Workers

### Which Cloudflare product(s) does this pertain to?

Workers/Other

### What version of `Wrangler` are you using?

2.15.0

### What operating system are you using?

Linux

### Describe the Bug

Hey there! While developing an internal API framework for our Workers, I ran into an issue publishing a Worker using a class instance. Right now, Cloudflare's API refuses to accept that we're exporting a fetch function even though this code works in both Miniflare and wrangler dev:

```js
class Test {
async fetch(req, env, ctx) {
return new Response("Hello World")
}
}

const test = new Test()

export default test
```

Yields:
✘ [ERROR] A request to the Cloudflare API (/accounts//workers/scripts/mre) failed.
`The uploaded script has no registered event handlers. [code: 10068]`