Add /jumptodate slash command (#7372)

Fix https://github.com/vector-im/element-web/issues/7677

Utilizing MSC3030: https://github.com/matrix-org/matrix-doc/pull/3030

Experimental Synapse implementation added in https://github.com/matrix-org/synapse/pull/9445

---

Jump to date headers are being worked on in https://github.com/matrix-org/matrix-react-sdk/pull/7339

src/settings/Settings.tsx

@@ -367,6 +367,15 @@ export const SETTINGS: {[setting: string]: ISetting} = {
         displayName: _td("New spotlight search experience"),
         default: false,
     },
+    "feature_jump_to_date": {
+        // We purposely leave out `isFeature: true` so it doesn't show in Labs
+        // by default. We will conditionally show it depending on whether we can
+        // detect MSC3030 support (see LabUserSettingsTab.tsx).
+        // labsGroup: LabGroup.Messaging,
+        displayName: _td("Jump to date (adds /jumptodate)"),
+        supportedLevels: LEVELS_FEATURE,
+        default: false,
+    },
     "RoomList.backgroundImage": {
         supportedLevels: LEVELS_ACCOUNT_SETTINGS,
         default: null,