ci: turn boilerplate into composite action

.github/actions/setup/action.yml

@@ -0,0 +1,19 @@
+name: Setup
+
+runs:
+  using: composite
+  steps:
+    - name: Install node
+      uses: actions/setup-node@v1
+      with:
+        node-version: 16
+    - name: Cache
+      uses: actions/cache@v2
+      id: cache
+      with:
+        path: '**/node_modules'
+        key: yarn-v1-${{ hashFiles('**/yarn.lock') }}
+    - name: Install
+      run: yarn --immutable
+      shell: bash
+      if: steps.cache.outputs.cache-hit != 'true'