Add CSS class for calculated CSS placement on Menu

[flippidippi]

Currently there is no out-of-the-box way to style the Menu/Control according to CoercedMenuPlacement when sending a classNamePrefix. This would help when users want to round the "outside" corners of menu. Depending on if the Menu is placed above or below, the rounded corners may need to be the bottom or top.

I think there could be a class on the Menu, like react-select__menu-bottom and ``react-select__menu-top` that would help solve this.