[display] show first argument of static extension methods in hover

@nadako recently brought up that the first argument of static extension methods should be shown in hover hints because it sometimes contains valuable information, for instance if there's a type parameter involved.

It probably has to stay as-is for completion/signature help... Although, technically Haxe could probably just start with an active parameter of 1 / skip the first argument. That would allow to still have consistent behavior across the display modes.