Remove .default & .resetIdCounter from index.js

[Andarist]

Problem description:
Such static property assignments prevent tree-shaking (which is still relevant in case of downshift in case it gets imported by some some unused part of the code, i.e. some ui-kit library which index.js reexports bunch of stuff):

https://github.com/paypal/downshift/blob/d2df6ec05198609ec5abbbad92bdc91dcac3b8c9/src/index.js#L9-L10

Not mentioning that adding .default is not proper way of supporting a default import declaration. We've discussed this once here - kentcdodds/kcd-scripts#28

Suggested solution:

1. Remove .default and use exports: 'named' in rollup.config.js or create a separate file for cjs that could wrap the original entry to provide .default
2. make .resetIdCounter a separate export and not a static property of Downshift class

[Andarist]

@kentcdodds if you specify which one of those:

• remove .default and use exports: 'named' in rollup.config.js
• create a separate file for cjs that could wrap the original entry to provide .default
  would u like to have done, then I can work on this one - although not sure if I manage to do that before the weekend and I'm not sure how fast you'd like to release the new version

[kentcdodds]

No worries. This change is pretty easy to make so I'll go ahead and do it myself.

I'm going to do:

remove .default and use exports: 'named' in rollup.config.js

Considering the rollup.config.js is already using exports: 'named' 👍

[kentcdodds]

Fixed!