fix: raise an error during wizening for async functions given to addEventListener

[JakeChampion]

resolves #687

Below is what what it looks like if you try and register an async fetchevent handler

❯ fastly compute serve
✓ Verifying fastly.toml
✓ Identifying package name
✓ Identifying toolchain
✗ Running [scripts.build]

INFO: Command output:

Error: addEventListener: Argument 2 should not be async function. Please rewrite this to be a synchronous function which returns a Response or a Promise of a Response.
Error: the wizer.initialize function trapped

The fetch event handler function has to be sync and not async because the Event Model itself is synchronous and you can register multiple FetchEventHandlers.
If you pass an async function as the FetchEventHandler and await before your call to event.responseWith - then the FetchEventHandler will return a Promise, when the FetchEventHandler returns anything (step 24.3.12 of Handle Fetch algorithm), the specification states we need to check if the the respond-with flag has been set, and it would not have been set - we then need to call the next registered FetchEventHandler - doing the same steps, until one calls event.respondWith or we have called them all.

This thread has some more context: w3c/ServiceWorker#836

[elliottt]

Awesome!