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/global.d.ts

@@ -1,13 +1,8 @@
-declare module "../tools/Split";
-declare module "../tools/Compress";
-declare module "../components/PageEditor";
-declare module "../components/Viewer";
 declare module "*.js";
 declare module '*.module.css';
-declare module 'pdfjs-dist';
 
 // Auto-generated icon set JSON import
-declare module '../assets/material-symbols-icons.json' {
+declare module 'assets/material-symbols-icons.json' {
   const value: {
     prefix: string;
     icons: Record<string, any>;
@@ -17,6 +12,4 @@ declare module '../assets/material-symbols-icons.json' {
   export default value;
 }
 
-declare module 'pdfjs-dist/legacy/build/pdf.mjs'
-
 export {};