Fix a couple of bugs in Model and codegen (#753)
- Child removal had a bug where parent field would be cleared in elements following a splice position
- Model merge would incorrectly merged two children with the same name but no ID even if other fields should discirminate
- Rename "MergeModels" to "MergedModel" since it used type-like rather than function-like naming
- Tweaked codegen of empty blocks to match Prettier configuration
- Reformatted comments in the files I touched
Fixes #752
