feat(nuxt): add initial server-side validation

This is a work-in-progress PR to add automatic server-side form validation based on re-rendering a formkit form and populating it with data.

It adds the following server-side utils:

validateFormkitData - this allows you to validate specific data, for a lower-level validation:

await validateFormkitData (event, '/path/form/was/submitted/from', 'form-id', { data: 'that-was-submitted' })

defineFormkitEventHandler - this automatically validates data being passed on an endpoint, for use with action="/api/endpoint" for formkit forms in future. (It shouldn't be used on its own.)
```
export default defineFormkitEventHandler(async (event) => {
 const { data } = await readBody(event)
 return { ...data, validated: true }
}, 'test-form')
```

Key points for review:

it seems that node.input is somewhat async still, as I have to await node.ledger.settled('validating') to get a valid state - this seems like a regression from when we previously paired on this @justin-schroeder.
In the test, there seems to be some kind of shared context between the requests for valid/invalid data - the test is flakey. I wonder if this is an issue with formkit internals?

Next steps:

Automatic assignment/extraction of ID when action="" is applied to a form, so no need for allowlisting in the event handler.
Handle form submission state (pending/loading spinner, automatic submission of form id, data + route).

Co-authored-by: Justin Schroeder justin@formkit.com

formkit / formkit