Add github action to test building

would also do actual tests, but I haven't written any :)

.github/workflows/test.yml

@@ -0,0 +1,23 @@
+name: Test
+on: [push]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        node-version: [18, 19, current]
+    steps:
+    - uses: actions/checkout@v3
+    - uses: pnpm/action-setup@v2
+      with:
+        version: 8
+    - name: Use Node.js ${{ matrix.node-version }}
+      uses: actions/setup-node@v3
+      with:
+        node-version: ${{ matrix.node-version }}
+        cache: 'pnpm'
+    - name: Install dependencies
+      run: pnpm install --recursive --include-workspace-root
+    - name: Build
+      run: pnpm run --recursive --include-workspace-root build