Virtual entry point

[mansona]

This PR is to move the assets/app-name.js entry point into virtual content. This means that it doesn't need to be done in Stage 2 any more

Thinks to do before this is merged:

• implement lazy routes
• delete old implementation (stage 2)
• update entry-point target (i.e. have the script in index.html actually target #embroider-entrypoint)
• implement static-app-paths config
• implement fix extension resolving for esbuild #1827 for the audit system
• move the CI to all use vite (and extract it to a new PR that could be merged first)