Make ! a builtin too, fixing "! -h"

UnLike other aliases (":.["), ! is special in the grammar but in the
few cases like "! -h" where we parse it as decorated statement they
are equals. Add it to the built in list, so the help argument works.

It can still be overridden, so this should not break anything.

share/functions/__fish_print_help.fish

@@ -1,5 +1,7 @@
 function __fish_print_help --description "Print help message for the specified fish function or builtin" --argument-names item error_message
     switch $item
+        case !
+            set item not
         case .
             set item source
         case :