fix: wait a tick before collecting form data for validation

[dummdidumm]

This way when you're doing

myForm.fields.foo.set('bar')
myForm.validate();

you'll validate on the up-to-date data

closes #14630

Don't think we need a test for this.

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-14631-svelte.vercel.app/

[hyunbinseo]

So they look like synchronous functions, but are actually async? That is magical.

[Rich-Harris]

set is synchronous but results in a DOM update that happens after a microtask (because DOM updates are batched). validate is asynchronous but you don't need to await it, it'll just update issues whenever it resolves