Adds feature flag for 5.0 redesign (#39802)

app/javascript/mastodon/utils/environment.ts

@@ -18,7 +18,7 @@ export function isServerFeatureEnabled(feature: ServerFeatures) {
   return initialState?.features.includes(feature) ?? false;
 }
 
-type ClientFeatures = never;
+type ClientFeatures = 'redesign';
 
 export function isClientFeatureEnabled(feature: ClientFeatures) {
   try {
@@ -30,3 +30,8 @@ export function isClientFeatureEnabled(feature: ClientFeatures) {
     return false;
   }
 }
+
+/* Checks if the 5.0 redesign features are enabled or not. */
+export function isRedesignEnabled() {
+  return isClientFeatureEnabled('redesign');
+}