Reference @sveltejs/kit types from generated config

[Rich-Harris]

Describe the problem

For TypeScript to see SvelteKit's ambient types ($app/paths and so on) you have to reference them:

/// <reference types="@sveltejs/kit" /> 

Typically this happens in src/app.d.ts. But there's lots of reasons why that might not be the case (e.g. a project was initialised without types). It'd be more reliable if the types were referenced automatically.

Describe the proposed solution

#5663 will introduce generated ambient types — once that's merged, it should be straightforward to reference the existing ambient types from there, or directly in the generated tsconfig.

Alternatives considered

No response

Importance

nice to have

Additional Information

No response