Tweak visibility of certain types

[Rich-Harris]

Describe the problem

There are some utility types that are exported from @sveltejs/kit but only so they can be imported by the generated $types.d.ts files. In the docs, they look rather chaotic:

Describe the proposed solution

I'm not really sure what to do with these — they are technically public types, but it would be nice if they weren't.

Alternatives considered

No response

Importance

nice to have

Additional Information

No response