Add a shared NotificationCountBadge view

Extract the unread notification count capsule from the room list cell into
its own view so it can be reused elsewhere. It caps the displayed value at
99+ and comes in a medium size (used by the room list) and a small size.

PreviewTests/Sources/GeneratedPreviewTests.swift

@@ -711,6 +711,13 @@ extension PreviewTests {
         }
     }
 
+    @Test
+    func notificationCountBadge() async throws {
+        for (index, preview) in NotificationCountBadge_Previews._allPreviews.enumerated() {
+            try await assertSnapshots(matching: preview, step: index)
+        }
+    }
+
     @Test
     func notificationPermissionsScreen() async throws {
         for (index, preview) in NotificationPermissionsScreen_Previews._allPreviews.enumerated() {

PreviewTests/Sources/__Snapshots__/PreviewTests/notificationCountBadge.iPad-en-GB-0.png

@@ -0,0 +1,3 @@
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+oid sha256:e1b9fdaf8ac1653153e78faa59162c930af5e5f6d647293ea907840f436f97b8
+size 73171

PreviewTests/Sources/__Snapshots__/PreviewTests/notificationCountBadge.iPad-pseudo-0.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:cdd5c07929cc8ac34ba855e4143f2ab1c23bde0c0780acfdf885e50c486733dc
+size 74808

PreviewTests/Sources/__Snapshots__/PreviewTests/notificationCountBadge.iPhone-en-GB-0.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:09a997528760f172b9b41994487e309e8958e7dd2404585bc7535a9f0f0b160a
+size 33306

PreviewTests/Sources/__Snapshots__/PreviewTests/notificationCountBadge.iPhone-pseudo-0.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:c058780bc218c210e7b1b12efd2838f332091103246cdebfb7a2f717dbfb2d9c
+size 34183