Make remix compatible with node's http module

[nicksrandall]

I propose we make a few minor changes to the @remix-run/express package to remove all "express" specific logic so that the module works with any http server framework based on node's http module. This will greatly improve Remix's compatibility with the rest of the node ecosystem.

Questions:

• If we do this, do we need to rename the @remix-run/express to something else? Maybe @remix-run/node-http?
• Should this module be extended to include logic for serving static assets since that is a requirement anyway?

Here is the associated PR: #3234