Create a minimum viable `ResolvedVc` type (#8661)
### Description
We want a way to more strongly guarantee that a `Vc` is resolved for the sake of "local" tasks.
This introduces it in a low-risk way with zero breaking changes. Eventually, we'd want to eliminate `.resolve()` or make `.resolve()` return `ResolvedVc` instead of using a separate `.to_resolved()` method.
Some rough notes about why here: https://www.notion.so/vercel/Resolved-Vcs-Vc-Lifetimes-Local-Vcs-and-Vc-Refcounts-49d666d3f9594017b5b312b87ddc5bff
### Testing Instructions
Tested as a part of #8678
