Stop exposing src directory on npm

[nolanlawson]

Our src is starting to contain more and more custom code that Rollup alters on the fly. I'd be more comfortable with building explicit ES module bundles on the same way we build CJS bundles.

[nolanlawson]

This would also be a good time to simplify the browser/node switching in our codebase using the trick in https://nolanlawson.com/2017/01/09/how-to-write-a-javascript-package-for-both-node-and-the-browser/

[stale]

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