Support merging document forks

[paulfitz]

There is easy and convenient UI to make experimental copies of a document, and to optionally "land" changes in that copy back on the original. But there isn't yet a convenient way to merge changes if the original diverges.

[dumblob]

This is enormously important (not in terms of urgency but in terms of design & plans). I envision this to become the single building block for local-first (offline-first) support in Grist.

This would help enormously with mobile support as mobile connections are very unstable and often not available at all (leaving aside the fact Grist is not yet fully optimized for smartphones :wink:).

Btw. check out https://josephg.com/blog/crdts-go-brrr/ if that'd suit certain merging use cases of Grist.