Added Auth.js based authentication

[NuroDev]

What's been changed?

• Added astro-auth integration
  • Created Auth.js config (auth.config.ts) with GitHub provider by default + Drizzle database adapter
  • Added dependencies
    • @auth/core
    • @auth/drizzle-adapter
    • auth-astro
  • Added Auth.js schemas to Drizzle
  • Added types for the new schemas added above
  • Added new migration to add schemas / tables required for authentication
  • Added sign out button to CommentForm component
  • Added new /login page
  • Homepage now redirects to /login if not logged in
• Added basic .example.env with template environment variables
• Added a defaultName prop to CommentForm component
• Added custom environment variable types to env.d.ts

[NuroDev]

I've just realised that the README.md may also need updating in relation to the environment variables.

Shall we add the REPLICA_URL, LITESTREAM_ACCESS_KEY_ID & LITESTREAM_SECRET_ACCESS_KEY environment variables to the .example.env? And/or update the README.md to tell people how to use the .example.env instead?