Add package typechecking to CI

Author: ultraq

.github/workflows/build.yml

@@ -6,7 +6,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        node-version: [18, 20]
+        node-version: [18, 20, 22]
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-node@v4
@@ -17,6 +17,7 @@ jobs:
       - run: npm run lint
       - run: npm run build
       - run: npm test
+      - run: npm run typecheck
       - uses: codecov/codecov-action@v4
         with:
           directory: coverage/