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/viewer/Viewer.tsx

@@ -0,0 +1,17 @@
+import EmbedPdfViewer from '@app/components/viewer/EmbedPdfViewer';
+
+export interface ViewerProps {
+  sidebarsVisible: boolean;
+  setSidebarsVisible: (v: boolean) => void;
+  onClose?: () => void;
+  previewFile?: File | null;
+  activeFileIndex?: number;
+  setActiveFileIndex?: (index: number) => void;
+}
+
+const Viewer = (props: ViewerProps) => {
+  // Default to EmbedPDF viewer
+  return <EmbedPdfViewer {...props} />;
+};
+
+export default Viewer;