Add linting against the use of if function

The `if-function` deprecation warning only comes with Sass 1.95[^1],
and we don't want to update quite yet so we can keep tracking `mixed-decls`
warnings until we upgrade sass[^2].

Stylelint will help us keep track of our usage of the `if` function in the meantime.
Because our code is written in Scss, we need to enable the rule from `stylelint-scss`,
rather than the one provided by stylelint.

[^1]: https://sass-lang.com/documentation/breaking-changes/if-function/
[^2]: #6690

Author: romaricpascal

stylelint.config.js

@@ -209,6 +209,7 @@ module.exports = {
       'src',
       'cursor',
       '-webkit-appearance'
-    ]
+    ],
+    'scss/function-disallowed-list': ['if']
   }
 }