feat: custom Vite server, index.html, Slidev integration example

[galvez]

This adds two new configuration options to @fastify/vite:

• server: allows providing a custom Vite dev server instance
• prepareHtml: allows providing custom string to serve as index.html file

And a usage example demonstrating how to serve a slidev presentation.

[galvez]

Closing for the time being as this gets deprecated by the new Vite environment API.