fix(stylelint): Ignore Sass @-rules and directives

Related to 3881d0a which added the rule `at-rule-no-unknown` to stylelint-config-standard.

Author: mischah

.stylelintrc.json

@@ -2,6 +2,21 @@
   "extends": "stylelint-config-standard",
   "rules": {
    "indentation": "tab",
-   "number-leading-zero": ["always", {"severity": "warning"}]
+   "number-leading-zero": ["always", {"severity": "warning"}],
+   "at-rule-no-unknown": [ true, {
+      "ignoreAtRules": [
+        "extend",
+        "mixin",
+        "include",
+        "at-root",
+        "debug",
+        "warn",
+        "error",
+        "if",
+        "for",
+        "each",
+        "while"
+      ]
+    }]
  }
 }