[6.x] Fix sync/desync on localizable nested fields

[duncanmcclean]

This pull request fixes syncing/desyncing nested fields (like grid, group, replicator, etc) when localizable: true.

In v5, when a nested field's value was updated, it would emit events which would eventually make their way back up to the respective fieldtype component (eg. Grid.vue), which would emit an event of its own, and eventually update the container, causing the localizedFields state to be updated.

Publish Containers were completely rewritten in v6 — methods/state can be accessed by child components via injection, doing away with the need for emitting events all the way up the chain.

When a nested field's value is updated, its value is updated directly in the container's state without needing to go up the chain to the nested fieldtype component. However, this meant that the nested key was being added to the localizedFields array, rather than the key of the "root" field.

This PR ensures we pass the path of the "root" field to syncField() and desyncField() instead of the nested field's full path. Not 100% sure if it's the right solution, so if you can think of an alternative, let me know!

Fixes #13822