Encapsulate Kit-specific TypeScript config behind extends

[Rich-Harris]

Describe the problem

Follow-up to the discussion in #3064. The tsconfig.json that gets created with a new project has a variety of Kit-specific properties. It's fine, but it we add new config over time it won't be reflected in existing projects, and it feels a bit untidy/brittle to have things the kit.files.lib config be duplicated in the tsconfig.json paths config.

Describe the proposed solution

It might be better to have "extends": "[something]" where Kit-specific config is contained in [something]. The simplest version of this would be something along the lines of @sveltejs/kit/tsconfig, but that has the drawback of not being able to contain app-specific types (like aliases derived from svelte.config.js, or other generated types — #647). Extending from .svelte-kit/tsconfig.json is a possibility, but a) it's a bit ugly and b) it would only be created/updated when you were developing or building your app, which would mean squigglies in the meantime.

Alternatives considered

No response

Importance

nice to have

Additional Information

No response

[Rich-Harris]

closed via #4118