impl. npm cache

.github/workflows/main.yml

@@ -12,6 +12,15 @@ jobs:
       uses: actions/setup-node@v1
       with:
         node-version: '12.x'
+    - name: Cache dependencies
+      id: npm-cache
+      run: |
+        echo "::set-output name=dir::$(npm config get cache)"
+    - uses: actions/cache@v2
+      with:
+        path: ${{ steps.npm-cache.outputs.dir }}
+        key: ${{ runner.os }}-node-${{ hashFiles('**/package-lock.json') }}
+        restore-keys: ${{ runner.os }}-node
     - name: Install dependencies
       run: npm ci
     - name: Compile CSS