Move AfterContext to WorkStore (#70806)
We want to support `after()` to run after all work has completed.
Ideally in any scenario that runs the parent. If there's multiple units
of work (e.g. batched Actions) or inner work like `"use cache"` /
`unstable_cache`, they should ideally all run at the end to avoid
blocking the other work.
It's difficult to remember how to wire this up each time we shadow a
unit or work. It's simpler just to put this on WorkStore which is where
we already have all internal "after" like `pendingRevalidates` and
`pendingRevalidateWrites`.
This is dependent on having a [WorkStore in
Middleware](https://github.com/vercel/next.js/pull/70808/) where we
currently support `after()`. We do not support `after()` in `pages/`.
