Sync Diffs App Vuex stores with Pinia counterparts

What does this MR do and why?

This MR enables syncing Diffs App Vuex stores (notes, diffs, batch comments and mr notes) to their migrated Pinia counterparts.

This change is required to enable iterative component migration to Pinia (otherwise all the components would have to be migrated in a single MR).

Resolves Sync Diffs, Notes and Batch comments Vuex store... (#513711 - closed).

How to reproduce

1. Open any merge request with changes
2. Open Vue Dev Tools (version 6 is required)
3. Open Changes tab
4. Select a diff file in Vue Dev Tools
5. Start recording events
6. Add a draft comment to a diff file
7. Observe that Pinia state has changed in the Vue Dev Tools events