Add abstraction layer between pinia stores and extensions

#4018 adds the ability for extensions to add stores. This should be abstracted to prevent accidental overwrite.

Ideally, this could be namespaced per-extension.

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-4020-Add-abstraction-layer-between-pinia-stores-and-extensions-2036d73d365081dcbf82e9eb1162c19b) by [Unito](https://www.unito.io)

Review the changes introduced by #4018 and trace how extensions currently add Pinia stores. Define the store abstraction and its extension-namespacing behavior, then verify that extensions cannot accidentally overwrite one another’s stores.

Written by the indexing model from the issue text.