feat: add easing functions to react-spring

[joshuaellis]

Why

What

Checklist

• Documentation updated
• Demo added
• Ready to be merged

[vercel]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/pmndrs/react-spring-io/7UhU14tG6ynQkwqZQRy5tFSddC7g
✅ Preview: https://react-spring-io-git-feat-add-easing-functions-pmndrs.vercel.app

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit 2082539:

Sandbox	Source
spring-card	Configuration
pmndrs/react-spring	Configuration
spring-flip-card	Configuration
spring-slide	Configuration
spring-draggable-list	Configuration
pmndrs/react-spring	Configuration
pmndrs/react-spring	Configuration
spring-simple-transition	Configuration
spring-image-fade	Configuration
spring-list-reordering	Configuration
spring-masonry	Configuration
spring-animating-auto	Configuration
spring-multistage-transition	Configuration
spring-chain	Configuration
pmndrs/react-spring	Configuration
spring-css-keyframes	Configuration
spring-tree	Configuration
spring-notification-hub	Configuration
initial-rocket	Configuration
spring-notification-hub	Configuration
spring-notification-hub	Configuration