Add `remix` package

[mjackson]

The remix package is a rollup of all the @remix-run/* packages. It uses package.json "subpath" exports to expose different pieces of functionality in Remix. There should be very little implementation code in the remix package. Most (if not all) implementation belongs in some dependency package.

I think the key to getting the exports right is to build several different prototypes of apps that use Remix 3, with various runtime constraints. At a minimum, we should have prototypes for:

• Multi-page app (MPA) with server-side rendering (SSR)
• Single-page app (SPA) with client-side rendering (CSR)
• Modern SSR with hydration
• Static site generation (SSG)
• Progressive web app (PWA), with offline capabilities

It should be possible to demonstrate all 5 in a single prototype like our current bookstore demo. But that's a large app and may be too unwieldy for folks to get a feel for everything, so a few simple prototypes that stick to one paradigm could also be useful.