Rename `Mention` to `IntentionalMention` for clarity
Remove dead code, there is no intentional mention for Room or RoomAlias.
Rename `IntentionalMention.AtRoom` to `IntentionalMention.Room` to match Rust naming
* Use ComposablePreviewScanner to rework how screenshot testing works
* Add test sharding
* Update screenshots
* Fixes for Element Gallery
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Co-authored-by: ElementBot <benoitm+elementbot@element.io>
Co-authored-by: Benoit Marty <benoit@matrix.org>