Change envPrefix from VITE_ prefix to PUBLIC_

[jasongitmail]

I imagine Vite uses the VITE_ prefix for public vars bc won't conflict with a user's existing vars in the app. But since TanStackStart is the application itself, it can make a decision based on DX.

PUBLIC_ is much more intuitive and explicit about implications of using the prefix.

SvelteKit make this change and it really makes the intention of each env var more immediately clear.

export default defineConfig({
  // ...
  envPrefix: 'PUBLIC_', 
});

https://vite.dev/config/shared-options.html#envprefix

(I am aware I can change it in my app :) But also seems a useful change for the framework DX, docs, & future users