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

.changeset/beige-shirts-work.md

@@ -0,0 +1,8 @@
+---
+"@medusajs/admin-vite-plugin": patch
+"@medusajs/admin-bundler": patch
+"@medusajs/admin-shared": patch
+"@medusajs/dashboard": patch
+---
+
+feat(admin-\*,dashboard): add dashboard i18n extensions