breaking: warn on self-closing non-void HTML tags

[Rich-Harris]

Closes #11052. It might be worth us creating a npx svelte-migrate self-closing-tags utility before releasing this

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🦋 Changeset detected

Latest commit: bc8232b

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[Rich-Harris]

Added a migration helper sveltejs/kit#12102

[Rich-Harris]

marking draft until sveltejs/kit#12102 (which is blocked on some prettier nonsense) can be merged

[brunnerh]

Wouldn't it be good to have something like "This acts as an opening tag." in the warning?

[reckart]

@Rich-Harris am I missing something or is this PR breaking using Svelte in XHTML contexts? It seems to be no longer possible now to inject a Svelte component e.g. with an <input> element into an XHTML document because the browser says that <input> without a closing tag or self-closing is not valid XML. Also, Svelte says that I am not allowed to make it valid XML on my own by adding a closing tag to <input>. And if I make <input/> self closing, Svelte simply renders it as <input> ignoring my wish.

[Rich-Harris]

No, it's unrelated. This PR is purely about authoring, not output