The script will never generate a response: worker-build should emit all code inside a closure in a fetch function

### Is there an existing issue for this?

- [X] I have searched the existing issues

### What version of `workers-rs` are you using?

0.0.0

### Describe the bug

Currently, the emitted code for `worker-build` has everything defined on the top-level.

I'm not sure why, but for some reason variables defined in the top level are re-used across requests.

I'm using the [`cached` crate](https://crates.io/crates/cached) in my project, and because the wasm memory is re-used across requests - it stops working and hangs with `The script will never generate a response.`

I'm not sure if this is the intended behaviour, re-using the wasm memory across requests - maybe it was done for perf reasons?

But anyways I've ran into this issue of randomly hanging workers loads of times, and it's really annoying. I think the default should be not to re-use memory across requests

### Steps To Reproduce

_No response_