[FR] Manual sync conflict resolution

### Description

I've noticed that with cloud sync between two devices, Appflowy tries its best to merge documents together when conflicts arise.

From my testing, I've found that it handles additions well enough. Combining additions to already edited documents when syncing from another device. But when text is removed on one device but edited on another, this can result in the entire portion of text being deleted once both devices are synced. This leaves me worried about potential data loss in some cases, as my phone is often left without internet and I often forgot to open and resync my old note taking app before making edits on my desktop in the past.

A system for manual resolution of conflicts by the user would provide the greatest peace of mind for this scenario.

### Impact

Any users using Appflowy on multiple devices with syncing enabled between them.

### Additional Context

Handling it the same way as Joplin would be perfect: If during a sync, it notices a document it wants to upload to the cloud has already been edited by another device, it moves its local version into a Conflicts folder, then downloads the latest synced copy in its place. From there, the user can compare and copy text from the conflicting version, edit the most recent version as they wish, and then delete the conflict copy.