Enforce double-colon notation for CSS pseudo-elements

These changes are part of https://phabricator.services.mozilla.com/D170496, and thanks to a Stylelint rule we can both enforce and fix this automatically; see also https://stylelint.io/user-guide/rules/selector-pseudo-element-colon-notation/

.stylelintrc

@@ -14,6 +14,7 @@
     "length-zero-no-unit": [true, {
       ignore: ["custom-properties"]
     }],
+    "selector-pseudo-element-colon-notation": "double",
     "shorthand-property-no-redundant-values": true,
   },
 }