build: Integrate webpack into build script

- Add `webpack` as a devDependency in `package.json`.
- Update `build-bookmarklet.sh` to run `npm install --silent` and `npx webpack` automatically, ensuring the required `dist/kantine.bundle.js` is generated before packaging.

Co-authored-by: TauNeutrino <1600410+TauNeutrino@users.noreply.github.com>

package.json

@@ -1,6 +1,7 @@
 {
   "devDependencies": {
     "jsdom": "^28.1.0",
+    "webpack": "^5.105.4",
     "webpack-cli": "^6.0.1"
   }
 }