[Popover] Migrate createSlideStyles to Fade motion component

[robertpenner]

createSlideStyles generates CSS keyframes that is used in Popover, MenuPopover, and about a dozen components that import them:

export function createSlideStyles(mainAxis: number): GriffelStyle {
  const fadeIn = {
    from: {
      opacity: 0,
    },
    to: {
      opacity: 1,
    },
  };
  // slide keyframes
  ...

Move the opacity transition to a Fade instance.