package.json: Change main to esnext

[ockham]

Reboot of #174. Don't merge yet!

Copying/updating #174's PR desc:

---

To better denote we're dealing with untranspiled code.

Companion Calypso PR: Automattic/wp-calypso#19698

To provide a bit of an explanation -- esnext is a custom field.

• We'll specify it as a possible entry point for webpack to watch out for here.
• OTOH, we use it (as opposed to main) as a flag to indicate that this is a package's entrypoint for untranspiled code here.

This is a strategy found at http://2ality.com/2017/06/pkg-esnext.html.

Suggested order:

• Merge this
• Release new version
• Calypso: Include notifications-panel version bump with Framework: Transpile untranspiled node_modules, Take Two wp-calypso#19698
• Test Framework: Transpile untranspiled node_modules, Take Two wp-calypso#19698
• Merge Framework: Transpile untranspiled node_modules, Take Two wp-calypso#19698

[dmsnell]

Does this need a version bump? Could we instead just add the esnext property and keep main?

[ockham]

I think by convention, main implies ES5 code, whereas it's esnext (or module) that means untranspiled code.

[kwight]

@ockham Anything we need here to move this forward?

[ockham]

@kwight Automattic/wp-calypso#19698 needs more testing -- @dmsnell discovered that right now it actually bloat bundle sizes rather than shrinking them.

I'm seeing more and more npms published with untranspiled code included, so IMO it'd be a really nice feature to be able to import that code OOTB...