Allow CDN backend to be customized

Currently unpkg.com is always used as the CDN backing external dependency imports.

The end-user should be able to change the CDN backend.

They should also be able to change the backend on a package-by-package basis. For example, on lit.dev it may be preferable to serve Lit itself from our own domain, but still allow other dependencies to come from unpkg.

A per-package "archive" format would also be great for performance. For example we could configure all of the Lit core packages (lit + lit-html + reactive-element) to be provided by a single JSON file so that most all of our examples only require 1 fetch for all dependencies.