Make merge conflicts occur on core objects rather than JSON blobs

[fuzyll]

Right now, we perform merge conflicts on JSON blobs pulled directly out of the snapshot we're merging from. This has a whole host of limitations associated with it, including:

• Support nested structure merging #5721
• Merging types breaks refs on conflicts #5722
• Need to merge type_lib_mapping #5723
• Need to handle large merge conflicts more gracefully #5724

We should instead make merge conflicts occur on core objects (falling back to JSON when necessary) so that our merge conflicts can be improved.