Add `unist-builder` utility to `remark` tranform

Currently when using the `remark` transform, the available utilities are:

- `unist-util-is`
- `unist-util-visit`
- `unist-util-visit-parents`

These are some of the most common utilities for `remark` transforms and I think they're a good set. However, building transforms for `remark` could be much more useful if [`unist-builder`](https://github.com/syntax-tree/unist-builder) was made available.

This utility makes it easy to construct new trees using a [`hyperscript`](https://github.com/hyperhype/hyperscript)-like syntax. Creating new trees currently has to be done by hand which can be quite time-consuming.