Create es-modules branch on upstream

[bennypowers]

The purpose of this PR is to move work on the es-modules branch from my fork to the main repo, in order to facilitate collaboration.

[karthikiyengar]

@bennypowers - I was wondering what the status of this PR is. Anything I could do to help get this to master?

[bennypowers]

I would love to revive this project.

Last I recalled this was slated for the next breaking change, but I'm not sure when that Is @evilsoft can help estimate?

I made some attempts at publishing an esm build of crocks, but they've fallen short for the time being (see esm-bundle/crocks#11 (comment))

So I'd really like to see crocks shipping esm sooner rather than later, as it remains a blocker for some at apollo-elements/apollo-elements#57

If there's consensus, we can try a non-breaking in-place transform by undertaking the following steps:

1. Rewrite all src to es modules, as per this PR
2. Use rollup to output CJS in-place as *.cjs files. e.g. under predicates, we'd have:

• hasProp.js - es module
• hasProp.cjs - cjs module
• hasProp.spec.cjs - cjs tests

3. Publish a point release

That last point I'm not 100% sure about. It may require (pun!) cjs-only users to require('crocks/**/*.cjs') with extension. We can experiment and try it out.

Having poked around a little bit, we could supply a snippet, or a side-effecting module which registers .cjs for older node versions in require.extensions