Drop support for older browsers and umd

This release changes exports to only provide an ESM package with no transpilation.

Add 'stop' function

• tween now returns a function that can be called to stop tweening

const stop = tween(0, 100, v => { animatedValue = v })
stop()

Update builds

This release changes a few minor things about the builds provided by this module:

• Typescript has been dropped, as it was providing almost no benefit.
• The UMD build has been renamed to femtoTween.umd.js instead of just femtoTween.js to be more explicit about its content.
• Sourcemaps are no longer provided for the esm build, because they were pointless.