Use native Object.assign instead of xtend

[jimmywarting]

xtend can easily be replaced with Object.assign

This reduce the dependencies and possible also avoid duplicated versions where some package don't use the same version range for the xtend package

// immutable
Object.assign({}, a, b)

// mutable
Object.assign(a, b)

[wooorm]

Let’s discuss over on https://github.com/syntax-tree/hast-util-from-parse5/issues/8.