style: Add a '-moz-menulist-button' value to '-moz-appearance'.

The '-moz-menulist-button' value currently behavies identically to the
'menulist-button' value.  This is not implemented as an alias because later
patches in this patch series will change the behavior of our pre-existing
'menulist-button' value to more closely match what Chrome does.

Bug: 1428676
Reviewed-by: emilio

components/style/properties/longhands/box.mako.rs

@@ -515,7 +515,6 @@ ${helpers.predefined_type(
     products="gecko",
     alias="-webkit-appearance:layout.css.webkit-appearance.enabled",
     spec="Nonstandard (https://developer.mozilla.org/en-US/docs/Web/CSS/-moz-appearance)",
-    needs_context=False,
     animation_value_type="discrete",
 )}