Support rendering React components to a string on the server

[chriskuech]

Goals

Render react components to an HTML string imperatively on the server, such as to render an email template or render the content of a PDF.

Non-Goals

Rendering a component to HTML in response to an HTTP request (ie normal NextJS rendering scenario)

Background

This is what I'm currently doing to access ReactDOM Server, which seems very hacky and unsustainable.

Proposal

Expose ReactDOM Server as a normal package that can use create-next-app's chosen module system (esnext, not cjs), without "legacy" in the path and with proper TS typings.