Formalize top-level ES exports

Currently we only ship CommonJS versions of all packages. However we might want to ship them as ESM in the future (https://github.com/facebook/react/issues/10021). We can't quite easily do this because we haven't really decided on what top-level ES exports would look like from each package. For example, does `react` have a bunch of named exports, but also a default export called `React`? Should we encourage people to `import *` for better tree shaking? What about `react-test-renderer/shallow` that currently exports a class (and thus would start failing in Node were it converted to be a default export)?