Add labs flag for default open right panel (#7618)

src/settings/Settings.tsx

@@ -366,6 +366,13 @@ export const SETTINGS: {[setting: string]: ISetting} = {
         displayName: _td("New spotlight search experience"),
         default: false,
     },
+    "feature_right_panel_default_open": {
+        isFeature: true,
+        labsGroup: LabGroup.Rooms,
+        supportedLevels: LEVELS_FEATURE,
+        displayName: _td("Right panel stays open (defaults to room member list)"),
+        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