Add latest stylelint rule

Author: philwareham

.stylelintrc.yml

@@ -198,6 +198,9 @@ rules:
   # Specify a blacklist of disallowed functions.
   #function-blacklist:
 
+  # Disallow an invalid expression within `calc` functions.
+  function-calc-no-invalid: true
+
   # Disallow an unspaced operator within `calc` functions.
   function-calc-no-unspaced-operator: true