Include Penner Easing equations in easing.css

[gordonbrander]

Robert Penner's easing equations are a battle-tested set of Bézier curves for plush animations:

• Quadratic
• Cubic
• Quartic
• Quintic
• Sine
• Exponential
• Circular

E.g. easeInOutSine easeOutExpo, etc.

These date back to the Flash days and make up a common default "kit" for easing (with the addition of springs).

Two CSS implementations:

• https://matthewlein.com/tools/ceaser
• https://gist.github.com/amadeus/983364

[argyleink]

these were in the original set of easings for Open Props! they were leveraging this postcss plugin https://www.npmjs.com/package/postcss-easings

and i agree, these are battle tested and often the first set that folks get introduced to when exploring the potential curves… i just always found their names confusing as hell to remember while also never really loving the results. BUT, just because I dont love them, doesnt mean they shouldnt be a part of this CSS variables collection 👍🏻

+1 to adding them back so the collection is the classic "kit" plus moarrrr