Add purely static export

[arctic-hen7]

Is your feature request related to a problem? Please describe.
Perseus is currently not technically JAMstack because it doesn't support eliminating server-side code. Many Perseus sites only rely on build-time rendering strategies, and performance would be improved if we could export to purely static files.

Describe the solution you'd like
The Perseus CLI should have an export command that creates a purely static set of files that can be served from anywhere, like a CDN, without any dependency on a server.

Describe alternatives you've considered
Zola is a great alternative in the meantime, but being able to have this work with Perseus would be a huge plus, and would get the project even with NextJS on JAMstack support.

Additional context
Add any other context or screenshots about the feature request here.

[arctic-hen7]

In theory, this should be able to be done by adding an extension of the build process for exporting, and shouldn't actually need any changes to the client-side app shell.

[arctic-hen7]

Okay, one slight pitfall of any approach to this is that we have to write the initial load files as .html files, which means whatever web server is used has to strip those extensions. For example, this causes things to break with Python's http.server. Somewhat ironically, Vercel's serve works perfectly! This problem should be very easy to solve though for whatever custom server ends up being set up.