Improve internal waitUntil utility (#56720)
⚠️ This is an internal API and will be removed soon. Please do not use.
Refactors #56404 to have a better internal API used by both Edge SSR and Edge Route Handlers.
This new API can buffer non-synchronously created "waitUntil"s even after the response has been returned, just need to make sure that there's at least one "waitUntil" queued. E.g.:
```js
async function handler() {
internal_runWithWaitUntil(async () => { // ← no await
await taskA()
internal_runWithWaitUntil(async () => { // ← no await
await longRunningTaskB()
})
await taskC()
})
return Response(...)
}
```
Internally, the "waitUntil" promise will resolve after all tasks are finished.
cc @ijjk @cramforce @feedthejim as we've synced about some of the details here. Not using ALS because of some promise-related issues.
