Scripts: use commons action for setup

.forgejo/snippets/setup.yml

@@ -0,0 +1,17 @@
+name: 'Setup'
+description: 'Checkout code, set up Nodejs run `npm ci`'
+runs:
+  using: 'composite'
+  steps:
+    - uses: https://source.mapcomplete.org/actions/checkout@v4
+
+    - name: Checkout up Node.js
+      uses: https://source.mapcomplete.org/actions/setup-node@v4
+      with:
+        node-version: "20"
+        cache: "npm"
+        cache-dependency-path: package-lock.json
+    - name: install deps
+      run: npm ci
+      shell: bash
+