Server feature for server crate

[yawn]

As discussed in Discord @ealmloff.

This shows how to include server-side specific functionality when using fullstack.

I believe it might be useful to change the name of the crate from server to something else though. It's getting even more confusing when also adding a feature with the same name.

[yawn]

Feel free to drop or squash the linebreaks commit. I found (the lack of) them confusing.

[yawn]

(deleted my previous comment)

My current thinking about a naming convention for mobile + web and fullstack that feels a bit more intuitive for me is the following:

• api crate: just the server_fn functions
• frontend crate
  • main entrypoint for web / mobile (e.g. for calling dioxus::fullstack::prelude::server_fn::client::set_server_url)
  • shared UI
• backend crate
  • main entrypoint and configuration for the fullstack server-side
  • axum customizations
• web / mobile crate
  • calls frontend or backend main, server-feature gated
  • builds the actual app out of frontend elements and customisations

All work in progress. WDYT?