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/components/fileManager/HiddenFileInput.tsx

@@ -0,0 +1,19 @@
+import React from 'react';
+import { useFileManagerContext } from '@app/contexts/FileManagerContext';
+
+const HiddenFileInput: React.FC = () => {
+  const { fileInputRef, onFileInputChange } = useFileManagerContext();
+
+  return (
+    <input
+      ref={fileInputRef}
+      type="file"
+      multiple={true}
+      onChange={onFileInputChange}
+      style={{ display: 'none' }}
+      data-testid="file-input"
+    />
+  );
+};
+
+export default HiddenFileInput;