Introduce an easy way to create errors from `onSubmit`

While discussing items we want to improve for a future v2, validation around onSubmit has come up a number of times.

As such, we're going to be backporting an API coming in v2 to the current version of Form (v1):

useForm({
   // ...
   onSubmit: async ({ value, createValidationError }) => {
     try {
        await callEndpoint(value)
        navigateToOverview()
     } catch (error) {
        if (error.code === 400) {
           return createValidationError({ form: `The endpoint had an issue: ${error.message}` }) 
        }
     }
   }
})

This work needs https://github.com/TanStack/form/pull/2128 to be merged first to avoid headaches with generics, but that should come shortly.

We want to hear from you if this API works/doesn't work for your needs and, if not, why not.

Thanks all!