Add support for cron triggers

[Randoooom]

Adds support for cron triggers (resolves #53).

I currently need some inspiration for handling an undefined scheduled event in the shim.mjs

[nilslice]

@Randoooom - this is great, thank you for working on adding scheduled event triggers.

I think we should still include waitUntil in the API, but it will require updating more code to include the ctx object into the function signature. I'd suggest taking a look at the docs for this function: https://developers.cloudflare.com/workers/runtime-apis/scheduled-event#syntax-module-worker

It might seem more complicated at first glance with the old implementation in workers-rs since it was leftover from a prior implementation where the EventListener format vs. modules

Let me know if you have any specific request for guidance or commentary on this while its in review. You can also join us on our Discord: https://discord.com/invite/cloudflaredev, where the team can more continuous conversation with you.

[Randoooom]

Oh yea, now I see I might be very stupid: I already implemented waitUntil but instead of using ctx I used the root object. I will keep working on it

[nilslice]

Not stupid at all - this is impressive work! Keep at it, and let me know how we can help.

[Randoooom]

Just resolved the generation of the shim.mjs.
I'm not sure if there is a better variant, but this is working at the moment:

    let shim_content = format!(
        r#"
import * as imports from "./{0}.mjs";

const fetch = imports.fetch;
const scheduled = imports.scheduled;

export * from "./{0}.mjs";
export default {{ fetch, scheduled }};
"#,
        bg_name
    );

[zebp]

Error handling for wait_until should be improved.

[zebp]

During my process of fixing this merge conflict, I accidentally managed to squash your commits. But this has been merged, thank you @Randoooom.