Add smoothstep

[Not-A-Normal-Robot]

https://en.wikipedia.org/wiki/Smoothstep

[scottmcm]

Alternatively, change some of the existing InOuts to be these Hermite functions

For example, this piecewise definition of easeInOutQuint is IMHO poor

easings.net/src/easings/easingsFunctions.ts

Lines 68 to 70 in 7200538

	easeInOutQuint: function (x) {
	return x < 0.5 ? 16 * x * x * x * x * x : 1 - pow(-2 * x + 2, 5) / 2;
	},

because that definition is really jerky: it jumps from +40 acceleration to -40 acceleration instantly https://www.desmos.com/calculator/vrnk8cfacz

If I'm bothering to go all the way to quintic, I'd expect it to be t³+3t³(1-t)+6t³(1-t)² so that the velocity, acceleration, and jerk are all continuous.

EDIT: If you want explicit formulas for the In and Out versions of Smooth(er)Step, you can find them in bevyengine/bevy#16957