feat(core): add experimental flag for journal (#5578)

packages/common/env/src/global.ts

@@ -29,6 +29,7 @@ export const runtimeFlagsSchema = z.object({
   enableEnhanceShareMode: z.boolean(),
   enablePayment: z.boolean(),
   enablePageHistory: z.boolean(),
+  enableJournal: z.boolean(),
   // this is for the electron app
   serverUrlPrefix: z.string(),
   enableMoveDatabase: z.boolean(),