add github action

.github/workflows/typecheck.yml

@@ -0,0 +1,24 @@
+name: Typecheck
+
+on:
+  pull_request:
+    branches:
+      - main
+
+jobs:
+  typecheck:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v2
+        with:
+          node-version: 18
+
+      - name: Install dependencies
+        run: yarn install --frozen-lockfile
+
+      - name: Run typecheck
+        run: yarn typecheck