Overhaul consumption model

[treshugart]

Currently there's main, module and browser fields in the package.json. We also have dist/cjs, dist/esm and dist/esnext. These are really confusing, but up to this point they've been necessary due to the decision to use native ES classes at the custom element level.

With the changes to Babel 7, and the plugins to transform HTMLElement extensions differently, we should look at offering a single entry point that's as simple as possible to use. We should also ensure that this is properly documented.

[morewry]

Just ran into a little issue with this trying to build skatejs@5.2.4 with Rollup; rollup-plugin-node-resolve doesn't support the esnext field that I needed to get at dist/esnext. Couldn't figure out for a bit why I was getting transpiled classes in my output that was ostensibly using the module entrypoint even though my Babel configuration should have been leaving them alone.

FWIW my current preference on this is to have a browser distribution transpiled with last 2 versions, not dead and a module distribution that presumes prior-year ES support.

[treshugart]

I'd like to just ship esmodules. Not sure if that's possible given usage in a node environment, though.