refactor: drop dependency on object assign

[ChristianMurphy]

Object.assign is now a native part of JavaScript
object-assign the package can be safely removed on Node and for most browsers (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign)

This reduces the bundled size significantly

[wooorm]

Looks good, but technically a breaking change.
We can sneak it into 2.0.1 maybe though, because the large ecosystem probably hasn’t updated yet.

Related to GH-1

[ChristianMurphy]

I agree technically breaking, though I anticipate it will be low impact since most adopters are using some for of build system, and polyfilling is a piece of most browser build systems, so the migration shouldn't be too difficult.