Fix failing unit tests

* This PR caused the Sidebar.test.ts file tests to fail because TransformStream is not found.
* TransformStream exists in the Node version I'm testing with (20), but it still isn't found by Jest
* Turns out it is a problem with Jest, and the workaround is the simple package jest-fixed-jsdom, which subclasses JSDOMEnvironment testing environment, placing this and several other dependencies in its global object.

* In package.json
  - add jest-fixed-jsdom as a devDependency

* In jest.config.cjs
  - change testEnvironment to jest-fixed-jsdom

package.json

@@ -52,6 +52,7 @@
     "@types/jest": "^29.5.14",
     "@types/node": "^22.7.5",
     "@types/shell-quote": "^1.7.5",
+    "jest-fixed-jsdom": "^0.0.9",
     "prettier": "3.3.3",
     "typescript": "^5.4.2"
   }