feat(admin-*,dashboard): add dashboard i18n extensions (#13763)

* virtual i18n module

* changeset

* fallback ns

fallback to the default "translation" ns if the key isnt found. Allows to use a single "useTranslation("customNs")" hook for both custom and medusa-provided keys

* simplify merges

* optional for backward compat

* fix HMR

* fix generated deepMerge

* test

packages/admin/dashboard/src/app.tsx

@@ -3,6 +3,7 @@ import { DashboardPlugin } from "./dashboard-app/types"
 
 import displayModule from "virtual:medusa/displays"
 import formModule from "virtual:medusa/forms"
+import i18nModule from "virtual:medusa/i18n"
 import menuItemModule from "virtual:medusa/menu-items"
 import routeModule from "virtual:medusa/routes"
 import widgetModule from "virtual:medusa/widgets"
@@ -15,6 +16,7 @@ const localPlugin = {
   displayModule,
   formModule,
   menuItemModule,
+  i18nModule,
 }
 
 interface AppProps {