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).

.stylelintrc.js

@@ -0,0 +1,17 @@
+/* global module */
+
+module.exports = {
+    "extends": "stylelint-config-standard",
+
+    "plugins": [
+        "stylelint-order"
+    ],
+
+    "rules": {
+        "order/order": [
+            "custom-properties",
+            "declarations"
+        ],
+        "indentation": 4
+    }
+};