feat: Start refactoring code base to be testable

Implement a spreadsheet service abstraction, GAS integration, and Jest testing setup.

package.json

@@ -7,15 +7,20 @@
   ],
   "scripts": {
     "build": "webpack --mode production",
-    "deploy": "clasp push"
+    "deploy": "clasp push",
+    "test": "jest"
   },
   "devDependencies": {
     "@types/google-apps-script": "^1.0.85",
+    "@types/jest": "^30.0.0",
     "copy-webpack-plugin": "^13.0.1",
     "gas-webpack-plugin": "^2.6.0",
     "graphql-tag": "^2.12.6",
+    "jest": "^29.7.0",
     "shopify-admin-api-typings": "github:beepmill/shopify-admin-api-typings",
+    "ts-jest": "^29.4.6",
     "ts-loader": "^9.5.1",
+    "ts-node": "^10.9.2",
     "webpack": "^5.96.1",
     "webpack-cli": "^5.1.4"
   },