[CI] Set up CI

.github/workflows/ci.yml

@@ -0,0 +1,34 @@
+name: CI
+on: [push, pull_request, workflow_dispatch]
+
+env:
+  FORCE_COLOR: true
+
+jobs:
+  build:
+    name: Typechecking & linting
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+      - uses: pnpm/action-setup@v2
+        with:
+          version: 9
+          run_install: false
+      - name: Install Node.JS
+        uses: actions/setup-node@v1
+        with:
+          node-version: '20.x'
+          cache: 'pnpm'
+
+      - name: Install dependencies
+        run: pnpm install --frozen-lockfile
+
+      - name: Typecheck
+        run: pnpm run check
+
+      - name: Check linting
+        run: pnpm run lint
+
+      - name: Check formatting
+        run: pnpm run check-fmt