Migrate to use Node's ES modules by default for both TS and JS users

[rwjblue]

See the docs here:

https://nodejs.org/api/esm.html

[scalvert]

Looking into this, it was very easy to convert all packages to output es modules vs. commonjs. It was largely achieved via tsconfig.json modification.

The main challenge preventing this from being implemented and shipped right now is the jest support. Support for esmodules in jest is experimental right now, and as such it wasn't easy to configure. We'll keep an eye on this, but this shouldn't block the 1.0 release, so removing this from the milestone for now.