fix: replace :-moz-placeholder-shown with :-moz-placeholder (#1532)

* fix: add blocker `-moz` prefix for `:placeholder-shown`

* docs: add `:-moz-placeholder`

Author: Marukome0743

lib/hacks/placeholder-shown.js

@@ -5,7 +5,9 @@ class PlaceholderShown extends Selector {
    * Return different selectors depend on prefix
    */
   prefixed(prefix) {
-    if (prefix === '-ms-') {
+    if (prefix === '-moz-') {
+      return ':-moz-placeholder'
+    } else if (prefix === '-ms-') {
       return ':-ms-input-placeholder'
     }
     return `:${prefix}placeholder-shown`