Add a resolved argument to `#[turbo_tasks::function]` (#68422)
### What & Why?
We want to enforce that values that functions return contain only
`ResolvedVc` and not `Vc` (though the outermost `Vc` is okay). We can do
that using the `ResolvedValue` marker trait
(https://github.com/vercel/turbo/pull/8678).
This PR allows enforcing that by passing a `resolved` argument to the
`#[turbo_tasks::function(...)]` macro.
This enforcement behavior is currently opt-in, but the goal is
eventually to make it opt-out, so that most functions can easily be
converted to use local tasks.
Bigger picture:
https://www.notion.so/vercel/Resolved-Vcs-Vc-Lifetimes-Local-Vcs-and-Vc-Refcounts-49d666d3f9594017b5b312b87ddc5bff
### How?
The key part of the macro is this bit:
```
fn assert_returns_resolved_value<
ReturnType,
Rv,
>() where
ReturnType: turbo_tasks::task::TaskOutput<Return = Vc<Rv>>,
Rv: turbo_tasks::ResolvedValue + Send,
{}
assert_returns_resolved_value::<#return_type, _>()
```
That creates no-op code that successfully compiles when the return value
(inside of the outermost `Vc`) is a `ResolvedValue`, but fails when it
isn't. This is the same trick that the [`static_assertions`
library](https://docs.rs/static_assertions/latest/static_assertions/macro.assert_type_eq_all.html)
uses.
### Test Plan
Lots of [trybuild](https://github.com/dtolnay/trybuild) tests!
```
cargo nextest r -p turbo-tasks-macros-tests
```
**Hint:** Use `TRYBUILD=overwrite` when intentionally changing the
tests.
