feat: make <svelte:option> customElement configuration's tag property optional (#12751)

[Theo-Steiner]

problem

As described in #12751, there are cases when users might want to use the advanced custom element configuration by <svelte:option customElement={...}> and still have control over when and how the custom element is defined with the customElements registry.

recap:

there are currently three possible options to set the tag name of a custom element in svelte

1. As a string value set to customElement: <svelte:options customElement="my-element" />
2. As a string value set to customElement.tag: <svelte:options customElement="{tag: 'my-element'}" />
3. Not setting it at all using <svelte:options />, but instead using customElements.define('my-element', MyElement.element);

fix: making tag optional in the configuration object as well

this PR makes it so that the tag property of the customElement configuration object now too is optional.
If a tag name is set, the element is registered automatically. If not, registering the element is left to the user.
This basically mirrors the behavior of how the non-object variant of customElement works

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: dfbeb21

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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[Rich-Harris]

thanks!