feat: provide PageProps, LayoutProps types

[rChaoz]

Closes #12726

This PR is a re-do of #12967 which went stale, addressing comments and properly updating the documentation.

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[changeset-bot]

🦋 Changeset detected

Latest commit: 117add2

The changes in this PR will be included in the next version bump.

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@sveltejs/kit	Minor

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[changeset-bot]

🦋 Changeset detected

Latest commit: 98f2626

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Minor

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[Rich-Harris]

preview: https://svelte-dev-git-preview-kit-13308-svelte.vercel.app/

this is an automated message

[teemingc]

Thank you! Tested this and it works well. Maybe another maintainer can take a look too.

[dummdidumm]

Thank you!