check treeshakeability in CI (#10861)

* check treeshakeability in CI

* copy-paste oops

.github/workflows/ci.yml

@@ -60,3 +60,18 @@ jobs:
       - name: build and check generated types
         if: (${{ success() }} || ${{ failure() }}) # ensures this step runs even if previous steps fail
         run: pnpm build && { [ "`git status --porcelain=v1`" == "" ] || (echo "Generated types have changed — please regenerate types locally and commit the changes after you have reviewed them"; git diff; exit 1); }
+
+  Treeshaking:
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - uses: actions/checkout@v3
+      - uses: pnpm/action-setup@v2.2.4
+      - uses: actions/setup-node@v3
+        with:
+          node-version: 18
+          cache: pnpm
+      - name: install
+        run: pnpm install --frozen-lockfile
+      - name: check
+        run: cd packages/svelte && node scripts/check-treeshakeability.js