Add fn keyword that added in PHP 7.4 (arrow function)

Author: zonuexe

Diff for: php-mode.el

@@ -625,6 +625,7 @@ PHP does not have an \"enum\"-like keyword."
     "endswitch"
     "endwhile"
     "eval"
+    "fn" ;; NOT c-lambda-kwds
     "global"
     "isset"
     "list"