Wip: [PoC] manifest v3 snippets

Beside this outstanding bug, which has been fixed in the meantime, we've already successfully experimented the usage of snippets with Manifest v3 API.

There are changes we're waiting for in the way snippets ship, as a module, but the logic can be already described, and be implemented, to drive this new way of running snippets through core, WebSDK, and the snippets module itself.

This issue simply demonstrates how to differentiate, and inject, snippets, once all changes will land.