Do not require `node-fetch` for browser targets

[AuHau]

Currently, the library solely depends on node-fetch for providing fetch primitives like Header class and so on.

We plan to use it in the browser context, where these primitives are provided by the runtime and we would like to avoid packing node-fetch because of it. I would like to provide a similar abstraction that https://github.com/sindresorhus/ky-universal uses for NodeJS and browser, utilizing the browser property in package.json.

[mdlavin]

Seems reasonable to me. Go for it