Restructure frontend code to allow for extensions (#4721)

# Description of Changes
Move frontend code into `core` folder and add infrastructure for
`proprietary` folder to include premium, non-OSS features

frontend/src/proprietary/data/useProprietaryToolRegistry.tsx

@@ -0,0 +1,13 @@
+import { useMemo } from "react";
+import { type ProprietaryToolRegistry } from "@app/data/toolsTaxonomy";
+
+/**
+ * Hook that provides the proprietary tool registry.
+ *
+ * This is the definition of all proprietary tools,
+ * and will be included in the main tool registry.
+ */
+export function useProprietaryToolRegistry(): ProprietaryToolRegistry {
+  return useMemo(() => ({
+  }), []);
+}