[feat] add App.PageData type

[dummdidumm]

Closes #5951

Marking as draft because the "layout data is required if one of the pages doesn't have a load function" check isn't implemented yet. If have an idea about storing the layout nodes in a map, which is updated by its pages, which also solves the O(n^2) problem, but my head needs a break so I'll revisit it later.
This all has gotten pretty sophisticated, which is why I'm also inclined to create some tests for it.

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Latest commit: 7fe3c4c

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@sveltejs/kit	Patch

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[Rich-Harris]

I was thinking we could add a children: PageNode[] property on PageNode and add children at the same time as we do this:

kit/packages/kit/src/core/sync/create_manifest_data/index.js

Line 292 in 9c7a6a9

current_node.parent = current_node = current_route.layout;

That would solve the O(n^2) problem and also ensure that the parent-child relationships were correct (which the current naive approach doesn't, since src/routes/foo/+layout.svelte treats src/routes/foo/bar/+page@.svelte as a child, incorrectly)

[Rich-Harris]

Just had some minor comments, but to the extent I was able to understand this (it's brain-bending stuff 😅) it looks awesome. Very excited for this one

[Rich-Harris]

this is a side-note, probably for a separate PR, but I've been wondering if we should just always spell out the params type, i.e.

PageLoad<{ id: string }>

rather than

PageLoad<RouteParams>

It's obviously more repetitive, but it means that if you hover over the params object you see this...

...instead of this, which is less useful:

[dummdidumm]

that's a good idea yeah, let's do this in a separate PR.