Style lint: Disallow shorthand & selectors (#8131)

* Impose and apply rule to prevent shorthand selectors

* Put one back to test the CI

* Put it back to pass CI

.stylelintrc.js

@@ -23,5 +23,10 @@ module.exports = {
             // https://github.com/vector-im/element-web/issues/10544
             "ignoreAtRules": ["define-mixin"],
         }],
+        // Disable `&_kind`-style selectors while our unused CSS approach is "Find & Replace All"
+        // rather than a CI thing. Shorthand selectors are harder to detect when searching for a
+        // class name. This regex is trying to *allow* anything except `&words`, such as `&::before`,
+        // `&.mx_Class`, etc.
+        "selector-nested-pattern": "^((&[ :.\\\[,])|([^&]))"
     }
 }