fix: form action submit function callback return type

[McAllaster]

Allow for form action submit function callbacks to be asynchronous functions.

Closes #13419

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Latest commit: 4b8c747

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@sveltejs/kit	Patch

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[McAllaster]

Apparently I was unfamiliar with the generated types, and pnpm check having a file output (for generating types) rather than just failing if there was something wrong. I'm going to convert this to a draft for now.

[elliott-with-the-longest-name-on-github]

Thanks!