Create separate browser and server-side builds

[overlookmotel]

Most of the code specifically for server-side is contained in the context injected on server side, and so not included in the main bundle.

However, could do a bit better in terms of minimizing the client-side bundle by producing both browser and server builds and using the relevant one using the browser key in package.json - like axios does.

In case that doesn't work for some users, could also allow browser and server builds to be imported directly from:

• react-lazy-data/browser
• react-lazy-data/node
• react-lazy-data/es/browser
• react-lazy-data/es/node