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/i18n/config.ts

@@ -0,0 +1,48 @@
+import i18n from 'i18next';
+import { initReactI18next } from 'react-i18next';
+import Backend from 'i18next-http-backend';
+
+i18n
+  .use(Backend)
+  .use(initReactI18next)
+  .init({
+    lng: 'en',
+    fallbackLng: 'en',
+    debug: false,
+    backend: {
+      loadPath: '/locales/{{lng}}/{{ns}}.json',
+    },
+    interpolation: {
+      escapeValue: false,
+    },
+    // For testing environment, provide fallback resources
+    resources: {
+      en: {
+        translation: {
+          'convert.selectSourceFormat': 'Select source file format',
+          'convert.selectTargetFormat': 'Select target file format',
+          'convert.selectFirst': 'Select a source format first',
+          'convert.imageOptions': 'Image Options:',
+          'convert.emailOptions': 'Email Options:',
+          'convert.colorType': 'Color Type',
+          'convert.dpi': 'DPI',
+          'convert.singleOrMultiple': 'Output',
+          'convert.emailNote': 'Email attachments and embedded images will be included',
+          'common.color': 'Color',
+          'common.grayscale': 'Grayscale', 
+          'common.blackWhite': 'Black & White',
+          'common.single': 'Single Image',
+          'common.multiple': 'Multiple Images',
+          'groups.document': 'Document',
+          'groups.spreadsheet': 'Spreadsheet',
+          'groups.presentation': 'Presentation',
+          'groups.image': 'Image',
+          'groups.web': 'Web',
+          'groups.text': 'Text',
+          'groups.email': 'Email'
+        }
+      }
+    }
+  });
+
+export default i18n;