Include `svelte.config.js` in `tsconfig.json`

### Describe the problem

We had to revert https://github.com/sveltejs/kit/pull/11886 in #11908 because it's a breaking change but it's probably good to include it

### Describe the proposed solution

Include it

### Alternatives considered

_No response_

### Importance

nice to have

### Additional Information

_No response_