Use native Object.assign instead of xtend

[xtend](https://github.com/Raynos/xtend) can easily be replaced with [Object.assign](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/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

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

// mutable
Object.assign(a, b)
```

Start by searching the repository for xtend imports and usages, then inspect the surrounding tests and run the existing test suite before changing anything. Done means the equivalent Object.assign behavior is used, xtend is no longer needed as a dependency, and the tests still pass.

Written by the indexing model from the issue text.