breaking: warn on self-closing non-void special elements

[jrmajor]

#11114, but for special elements, so that they're treated more consistently with HTML elements.

I should have opened an issue for that first, so please treat it like a place for discussion. No hard feelings if it doesn't get merged :)

This PR adds warnings only for <svelte:head> and <svelte:fragment>, since these are the only elements that don't really make sense without content.

<svelte:options>, <svelte:window>, <svelte:document> and <svelte:body> should never have content, so I've opened #11245 to treat them like void tags.

<slot> is deprecated anyway.

Other special elements sometimes feel like void tags, and sometimes not, so I didn't touch them:

<!-- components -->
<svelte:self />
<Foo />
<svelte:component this={"foo"} />

<!-- `this` can be bound to both void and non-void tags -->
<svelte:element this="dif" />

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[changeset-bot]

🦋 Changeset detected

Latest commit: 6ae0a93

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

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[jrmajor]

Closing, as decided in #11245.