feat: fix accessors and support migration of accessors

[paoloricciuti]

Svelte 5 rewrite

Closes #13453

This both fixes accessors when defined as exports and implement the migration from a component with <svelte:options accessors /> removing the accessors option and exporting all the props.

I've excluded props because whoever is migrating this component definitely was not accessing slots through accessors and they can always add it by hand if they want.

Please note that the Svelte codebase is currently being rewritten for Svelte 5. Changes should target Svelte 5, which lives on the default branch (main).

If your PR concerns Svelte 4 (including updates to svelte.dev.docs), please ensure the base branch is svelte-4 and not main.

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

[changeset-bot]

🦋 Changeset detected

Latest commit: de65e1c

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

This PR includes changesets to release 1 package

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svelte	Patch

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