[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.

[robertpenner]

• Discussion on refactor(Popover): migrate opacity transition to Fade component #33424 indicates that we should migrate the fade and slide together, rather than the fade separately, to avoid maintenance issues.
• The slide migration is high effort and high risk. The code has connections between JavaScript and DOM attributes that are tricky to read from motion components; there appears to a race condition to mitigate.
• Also, the slide uses an integration with a third-party library Floating UI.
• Fluent Eng flagged potential for regressions in Teams usage if we're not careful.
• Fluent Eng would like a motion slot created, which adds to the level of effort.

Thus, returning this to the backlog pending re-evaluation of priorities and strategy.