refactor(turbo-tasks): Add a way to directly construct a ResolvedVc (#69992)
Without these APIs, the only way to construct a `ResolvedVc` was:
```
Vc::cell(my_value).to_resolved().await?
```
However, we know when calling `Vc::cell` that the `Vc` must be resolved (since we just assigned it to a cell), so we can shortcut that and provide a better API:
```
ResolvedVc::cell(my_value)
```
For `#[turbo_tasks::value]`s, there's an equivalent `.resolved_cell` method.