Move to a single dependency

[mcrascal]

Today, when you initialize an evidence project you get a full sveltekit project built in your directory.

This is pretty gnarly for our users, and it makes managing dependencies tricky for us.

Future state

1. A working project should only require a pages directory with .md files, and the required evidence.config and database.config files.
2. The only dependency should be @evidence-dev/evidence. All of the other deps should be upstream of that.
3. The __layout, svelte config, app.css, api route etc. files that are currently dropped in the directory should no longer be required.
4. If a user did want to modify app.css or __layout (e.g. a new "theme") they should be able to add them directly to their directory and modify from there, with the build system preferring their additions.