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:

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

• 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',
		}),
	}}
/>

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.
• 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

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[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

[Rich-Harris]

Thank you. What's the purpose of allowing a function to be passed? What can you do that way that you can't do without the function?

[Bladesheng]

I think I added that in case the configuration logic would need to be more complex. But now I can't think of any scenarios where you would need more than just the object with some ternary operators. So it could be removed I guess.