Lazy checking of documents

[ejgallego]

The idea is to allow users to delay proof checking, and do so only under demand, for example when clicking a gutter.

There are many heuristics involved here, for example we could schedule for checking the proofs that are in view, but we need #49 and LSP 3.18 for that to be implemented relialby.

For now, we can start implementing the infrastructure to do so. The idea was that actually Coq upstream API would allow clients to re-implement vernacinterp, upstream diverged a bit on that matter, but I think we can still proceed with the original plan.

Basic lazy checking was implemented in #629

[ejgallego]

This requires parsing resumption so bumping milestone.