feat: adding basic unit tests and framework for them

package.json

@@ -15,11 +15,20 @@
   },
   "homepage": "https://github.com/example/my-extension#readme",
   "sideEffects": false,
+  "scripts": {
+    "test": "jest",
+    "test:watch": "jest --watch",
+    "test:coverage": "jest --coverage"
+  },
   "devDependencies": {
     "@girs/gjs": "4.0.0-beta.38",
     "@girs/gnome-shell": "49.0.1",
+    "@jest/globals": "^29.7.0",
+    "@types/jest": "^29.5.12",
     "eslint": "^9.36.0",
     "eslint-plugin-jsdoc": "^50.8.0",
+    "jest": "^29.7.0",
+    "ts-jest": "^29.1.2",
     "typescript": "^5.9.2"
   },
   "dependencies": {