react-smooth v4 "breaking changes"

[ckifer]

Doing this so we can remove the peer dependency on prop-types in recharts and bump our react peerDep

• prop-types upgrade minors and react-transition-group -> 4.x
• prop-types move to dependencies - consumers don't need to care about the version of this package
• remove prop-types from externals
• remove support for react < 16.8 - recharts breaks with <16.8 so upgrading this should be fine
• bump to 4.x

Note 3.x still breaks recharts so this can't be added right away

Testing: published with yalc and messed with it locally

[ckifer]

@PavelVanecek @nikolasrieble tagging as more of an FYI than anything, I'm just going to publish this so its there, we don't necessarily need to consume it right away (refactoring for v3 required anyways)