Shortcuts for non-linear interpolation

[LeaVerou]

Right now, we support custom functions for non-linear interpolation, which does make complex things possible.
However, it would be nice to also make simple things easy, by defining a few shortcuts.

Borrowing from CSS, these could be:

• Predefined functions for the CSS keywords (ease, ease-in, etc.)
• A function that takes in cubic-bezier() coordinates and returns a function
• Same for steps()

Outside of CSS, it would be good to see some use cases where non-linear interpolation is used, to see what primitives would facilitate them better.