Describe what a route's loaders resolve to

[pleek91]

Description

Fifth step of #805, stacked on #811. Types only.

RouteDataOf combines the loaders of every match, so a child sees an ancestor's data as its own, and flattens to the loader's data directly when the only loader in the tree is unnamed:

const user = createRoute({ name: 'user', path: '/user/[id]' })
  .addLoader(async (route) => api.getUser(route.params.id))

// data -> Promise<User>

const posts = createRoute({ parent: user, name: 'user.posts', path: '/posts' })
  .addLoader(async (route) => api.getPosts(route.params.id), { name: 'posts' })

// data.default -> Promise<User>
// data.posts   -> Promise<Post[]>

Data is always a promise, even for a synchronous loader. Loaders do not hold up rendering, so a component reading data cannot be given a settled value — making that uniform is cheaper than making it conditional.

It is carried by ResolvedRouteWithData rather than by ResolvedRoute, because only the route being navigated to has data. A route the router merely resolved — router.resolve, router.find, the to/from a hook receives — is not being loaded, so data on it could only be a promise that never settles. #814 gives it to the two surfaces that do have it: the current route, and a props getter's route.

Also worth a look: the unregistered case types data as Promise<unknown> | Record<string, Promise<unknown>> | undefined rather than just a record, because a route whose data is a bare promise has to satisfy the constraint.