Type generation extensibility hooks

Describe the problem

I want to create additional generated types like RequestEvent per route.

Describe the proposed solution

Something here

https://github.com/sveltejs/kit/blob/82a0e96e32fc0d1891d0cff2788d431a591710ab/packages/kit/src/core/sync/write_types/index.js#L181

That runs my own configured definition factories, the signature taking routes, route (and perhaps config, although why?) to append to declarations to be emitted when sveltekit types are generated

fn addDefinition({routes, route}) => {imports, declarations, exports} run and append them here

Maybe it could pick up a magic function from somewhere? Maybe a file that exists along the route directory structure? Falling back to tthe magic file at root or /app.

Alternatives considered

Script that runs and appends to each $types.d.ts after svelte has generated its types (unreliable)

Importance

would make my life easier

Additional Information

Allowing modification of core types is out of scope (probably not recommended at all?)

Start in packages/kit/src/core/sync/write_types/index.js at the referenced lines, and trace how route types and declarations are assembled. Clarify the hook location, configuration or magic-file discovery, and the definition-factory contract; done means configured per-route types such as RequestEvent are emitted without post-generation scripts.