feat: Add CI typecheck/test runs.

.github/workflows/checks.yml

@@ -0,0 +1,22 @@
+on:
+  pull:
+  push:
+    branches:
+      - main
+
+name: checks
+
+jobs:
+  checks:
+    strategy:
+      matrix:
+        command: [typecheck, test]
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 18
+          registry-url: 'https://registry.npmjs.org'
+      - run: npm ci
+      - run: npm run ${{ matrix.command }}