chore: give scroll and focus their own state and tests

[Nic-Polumeyv]

On top of #16980: scroll.js owns the after-navigation scroll and focus.js owns the blur and the focus reset, with finish_navigation the one place that sequences them. Whether the app moved focus during the update is now read from document.activeElement directly instead of a snapshot taken before the update, and a navigation that does not reset no longer scrolls to where it already is.

A focused SVG element is now blurred before the DOM update like any HTML element.

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 889a452:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/889a45281340aad710c21b4dc98d002cb5f11b43

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16983

[changeset-bot]

🦋 Changeset detected

Latest commit: 889a452

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	Patch

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-16983-svelte.vercel.app/

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