feat: allow passing ShadowRootInit to custom element shadow options

[Bladesheng]

Closes #17080

Allows you to change all options of ShadowRootInit when creating custom elements.

• You can still omit the shadow option entirely.
• You can still use shadow: 'open'.
• You can still use shadow: 'none'.
• You can now also pass the whole ShadowRootInit object, so that you can change all the settings of shadow root.
• You can now also pass function that returns ShadowRootInit object:

<svelte:options
	customElement={{
		tag: 'custom-element',
		shadow: () => ({
			mode: import.meta.env.DEV ? 'open' : 'closed',
			clonable: true,
			delegatesFocus: true,
			serializable: true,
			slotAssignment: 'manual',
		}),
	}}
/>

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• 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.
• If this PR changes code within packages/svelte/src, add a changeset (npx changeset).

Tests and linting

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

[changeset-bot]

🦋 Changeset detected

Latest commit: 8d32602

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

This PR includes changesets to release 1 package

Name	Type
svelte	Minor

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-17088-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@17088