Adds support for nested `unstable_after()`.
This pattern previously threw a "Not supported yet" error, but works
now:
```js
function MyComponent() {
after(() => asyncWork());
return <div>...</div>
}
async function asyncWork() {
after(() => { /* look ma, nesting!*/ })
// more stuff...
}
```
### Implementation notes
Switched `AfterContext` to use a proper promise queue
([`p-queue`](https://www.npmjs.com/package/p-queue)) instead of plain a
callback array to support adding more callbacks as we execute (i.e. from
nested `after`s). Used a package because I didn't want to reinvent the
wheel here.
As a nice bonus, `p-queue` lets us limit the concurrency of running
tasks if we're worried about resource consumption. **This PR doesn't do
that**, but it's very easy to add. That could be controlled via
`process.env.NEXT_AFTER_MAX_CONCURRENT_TASKS`, a next.config.js option
(`unstable_after: { maxConcurrentTasks: 5 }`), or something like that.
663488cf52