Feature: Portals

We need to implement a proof of concept for Portals in Mitosis very similar to React: some sort of mechanism that will allow users to render components at the root.

This is required for users to be able to do certain things like building Modals, popups, dialogs, etc.

In the case of React, it would be as simple as reusing `React.createPortal()` in the generated React code. What we really need to do is investigate all other frameworks to see how we can provide consistent functionality across all targets.