Move ES module build to es/

[taion]

This makes us consistent with Redux

[timdorr]

Let's make a note of this in the Changelog so that those living on the edge and building via these files know to update.

[ryanflorence]

isn't moving the directory of the lib's location a breaking change?

[ryanflorence]

oh, nvm, main solves it.

[taion]

jsnext:main, but yeah – the idea is that users should be importing this as 'react-router' anyway, not reaching into es/ manually.