prepare for kit@3

[manuel3108]

This is a non exhaustive list that we will need to add and remove stuff as we go.

sv create

• Templates should use kit@3
• get rid of svelte.config.js in favor of vite.config.js
• overhaul demo template to use remote functions if relevant, see create: update template to use remote functions and async #641

sv migrate

• migrate package.json and stuff like that to kit@3
• get rid of svelte.config.js in favor of vite.config.js
• allow migrating single files, in example Migrating a single component to Svelte 5 syntax? #1103
• make sv migrate the single source of truth for migrations, as vscode migration in example do not apply everything that was part of the sv migrate svelte-5 thingy, see Move mount/unmount migrations to rest of migration code? #1104

sv add

• check compatibility of relevant official add-ons
• get rid of $env/* imports in drizzle and better-auth