chore: cleanup storybook setup
- fixes build-storybook command which was missed when migrating from npm to pnpm
- move internal `@starlight/storybook-utils` package to be a dev dependency
- partially update `.storybook/main.ts` file in `/app` and `package/icons` to use absolute paths (done with storybook's
migration codemod)
- update storybook deps from v10.1.10 to v10.1.11