Allow silent notifications to be treated as notifications for notification collector

**Suggestion**

This is https://github.com/matrix-org/matrix-doc/issues/3447 / https://github.com/vector-im/element-web/issues/15025, I hate keeping notification sounds enabled as they distract me and break my flow getting things done. However currently I have to enable noisy notifications in Element Web for it to recognise notifications as notifications for it's 🔔 menu.

Start by reading Matrix spec issue 3447 and Element Web issue 15025, which provide the referenced context. Define the protocol change needed for silent notifications to be recognized by the notification collector, and confirm that the resulting specification clearly covers this behavior.

Written by the indexing model from the issue text.