Add new easing function generators: generateBack, generateElastic, generateSteps

[0xTHAC0]

Summary

Adds three new configurable easing generators to complement the existing generatePow, as requested in #507:

• generateBack(overshoot) — Back easing with custom overshoot (default 1.70158, matching the built-in Back). Higher values produce more dramatic overshoots.
• generateElastic(amplitude, period) — Elastic easing with configurable amplitude (≥1) and period. Useful for springy animations where you need fine control over the bounce.
• generateSteps(steps) — Discrete step easing that quantizes the animation into N steps. In variant jumps at the start of each interval, Out at the end, InOut combines both halves. Useful for sprite sheet animations and staircase effects.

All generators return EasingFunctionGroup (In/Out/InOut) and satisfy the standard edge-value contracts:

• f(0) = 0
• f(1) = 1
• InOut(0.5) = 0.5

Changes

• src/Easing.ts — added the three generator functions
• src/tests.ts — added 7 test cases covering edge values, discrete step behavior, midpoint constraints, and generateBack parity with default Back
• examples/03_graphs.html — added visual demos for the new generators

Usage

import { Easing } from '@tweenjs/tween.js'

// Back with stronger overshoot
const strongBack = Easing.generateBack(3)
tween.easing(strongBack.InOut)

// Elastic with more bounce
const springy = Easing.generateElastic(1.5, 0.4)
tween.easing(springy.Out)

// 4-step staircase
const steps = Easing.generateSteps(4)
tween.easing(steps.In)

Closes #507