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/useBaseUrl.ts

@@ -0,0 +1,6 @@
+import { useAppConfig } from '@app/contexts/AppConfigContext';
+
+export const useBaseUrl = (): string => {
+  const { config } = useAppConfig();
+  return config?.baseUrl || 'https://demo.stirlingpdf.com';
+};