[Fade] Migrate to new variant structure

[robertpenner]

1. Migrate Fade's variant definitions to the functional style as in Collapse.
2. Update Fade's variant customization example in Storybook.

Future Work

• Migrate Scale's usage of createPresenceComponentVariant.
• Migrate unit tests that reference createPresenceComponentVariant and overridePresenceMotion.
• Delete createPresenceComponentVariant and overridePresenceMotion as unused and obsolete.