Add linter for CSS files:

- Add Github Action.
- Update .gitignore.
- Add .stylelintignore for vendor related files.
- Fix format.css to match rules (includes hacks with @todo).

package.json

@@ -0,0 +1,7 @@
+{
+  "devDependencies": {
+    "stylelint": "^13.12.0",
+    "stylelint-config-standard": "^21.0.0",
+    "stylelint-order": "^4.1.0"
+  }
+}