feat(dashboard): Regions domain (#6534)

**What**
- Implements new Region domain design
- Adds new SplitView component for managing adding nested relations in FocusModals, eg. adding countries to a region.
- Adds new Combobox component for multi select fields in forms

**medusajs/ui**
- Fix styling of RadioGroup.Choicebox component

CLOSES CORE-1650, CORE-1671

packages/admin-next/dashboard/src/hooks/use-media-query.tsx

@@ -0,0 +1,14 @@
+import { useEffect, useState } from "react"
+
+export const useMediaQuery = (query: string) => {
+  const mediaQuery = window.matchMedia(query)
+  const [matches, setMatches] = useState(mediaQuery.matches)
+
+  useEffect(() => {
+    const handler = (e: MediaQueryListEvent) => setMatches(e.matches)
+    mediaQuery.addEventListener("change", handler)
+    return () => mediaQuery.removeEventListener("change", handler)
+  }, [mediaQuery])
+
+  return matches
+}