fix: make slide routes async, prevent crash on syntax errors

[KermanX]

This PR contains:

• No white screen when there is a syntax error in slides, and auto-reload when the error is solved.
• Resolves when I write '---' in slides.md file to add a page , this full page are reloaded, but I don't want to reload all pages, how to auto focus current page? #1355. Now adding and removing slides will not trigger a full reload.
• Refactors useNav, which used to be very ambiguous.
  • useNav used to mix the primary route and fixed route for exporting.
  • Now, it is renamed to useNavBase and useFixedNav (similar to the clicks context)
• Migrate to vite v5's HMR API, and give types to custom events.

Internal changes

• Slide components are now async. And a loading animation is added after 100ms. Not very sure about this animation.
• rawRoutes is renamed to slides, with reduced data.

[antfu]

Very solid PR as always, thanks!