Port configurable easing generators from #711 into this repository

[Copilot]

This brings the work from #711 onto a branch owned by tweenjs/tween.js so review and follow-up can continue here. It ports the new configurable easing generators and the supporting examples, typings, and test coverage needed to evaluate them in-repo.

• API additions

  • add Easing.generateBack(overshoot?)
  • add Easing.generateElastic(amplitude?, period?)
  • add Easing.generateSteps(steps?)
  • keep the generators aligned with existing easing contracts at 0, 1, and 0.5 for InOut

• Package surface

  • expose the new generators from src/Easing.ts
  • propagate the API into generated distribution bundles and TypeScript declarations

• Examples

  • extend examples/03_graphs.html with representative graphs for:
    • generateBack(3)
    • generateElastic(1.5, 0.4)
    • generateSteps(4)

• Coverage and follow-up cleanup

  • add assertions for default-parameter parity with Back, edge values, midpoint behavior, and stepped output semantics
  • make the existing Tween.end() final-value test deterministic by using an explicit start time

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

const back = Easing.generateBack(3)
const elastic = Easing.generateElastic(1.5, 0.4)
const steps = Easing.generateSteps(4)

tween.easing(back.InOut)
tween.easing(elastic.Out)
tween.easing(steps.In)