LibWeb: Prepare to run callback in host_enqueue_promise_job()

...and clean up afterwards, of course. Additionally to preparing to run
a script, we also prepare to run a callback here. This matches WebIDL's
invoke_callback() / call_user_object_operation() functions, and prevents
a crash in host_make_job_callback() when getting the incumbent settings
object.

Running the following JS no longer crashes after this change:

```js
new Promise((resolve) => {
    setTimeout(resolve, 0);
}).then(() => {
    return Promise.reject();
});
```

See further discussion/investigation here:
https://discord.com/channels/830522505605283862/830525031720943627/995019647214694511
https://discord.com/channels/830522505605283862/830525031720943627/1026824624358576158
https://discord.com/channels/830522505605283862/830525031720943627/1026922985581457458

Many thanks to Luke for doing the hard work here, tracking this down,
and suggesting the fix!

Co-authored-by: Luke Wilde <lukew@serenityos.org>

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