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/core/hooks/useFileHandler.ts

@@ -0,0 +1,17 @@
+import { useCallback } from 'react';
+import { useFileActions } from '@app/contexts/FileContext';
+
+export const useFileHandler = () => {
+  const { actions } = useFileActions();
+
+  const addFiles = useCallback(async (files: File[], options: { insertAfterPageId?: string; selectFiles?: boolean } = {}) => {
+    // Merge default options with passed options - passed options take precedence
+    const mergedOptions = { selectFiles: true, ...options };
+    // Let FileContext handle deduplication with quickKey logic
+    await actions.addFiles(files, mergedOptions);
+  }, [actions.addFiles]);
+
+  return {
+    addFiles,
+  };
+};