Add job to check GDScript format

.github/workflows/main.yml

@@ -311,3 +311,15 @@ jobs:
           exit 1
         fi
 
+  check-code-format:
+    name: 'Check Code Format'
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: actions/setup-python@v2
+      with:
+        python-version: 3.x
+    - name: GDFormat Check
+      run: |
+        python -m pip install -r requirements.txt
+        gdformat -c .