chore: clean up a11y analysis code

[Ocean-OS]

When reviewing #16340, #16341, and #16339 I realized how unorganized the code in packages/svelte/src/compiler/phases/2-analyze/visitors/shared/a11y.js was, so I split it into two files and cleaned up a few things.

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[changeset-bot]

🦋 Changeset detected

Latest commit: 21ed6c1

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

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-16345-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@16345

[dummdidumm]

"Few things" are from what I can see mainly if->switch? Anything else?
I think we should name it a11y/index.js not a11y/utils.js, otherwise looks good.

[Ocean-OS]

iirc that's all, I'll rename it in a bit