[FEAT] Remove Form Factory + Change React SSR Story

[crutchcorn]

This PR:

• Removes createFormFactory
• Introduces a new API for formOptions
• Introduces a new API called createServerValidate for SSR usage
• Updates React adapter tests
• Update Vue adapter
• Update Solid adapter
• Updates SSR docs to reflect new APIs
• Updates other docs to reflect new APIs

Together, this API looks like:

// shared-code.ts
import { formOptions } from '@tanstack/react-form/nextjs'

export const formOpts = formOptions({
  defaultValues: {
    firstName: '',
    age: 0,
  },
})

// client-component.tsx

/* ... */
const form = useForm({
  ...formOpts,
  transform: useTransform(
    (baseForm: FormApi<any, any>) => mergeForm(baseForm, state),
    [state],
  ),
})
/* ... */

// action.ts
'use server'

import { createServerValidate } from '@tanstack/react-form/nextjs'
import { formOpts } from './shared-code'

const serverValidate = createServerValidate({
  ...formOpts,
  onServerValidate: ({ value }) => {
    if (value.age < 12) {
      return 'Server validation: You must be at least 12 to sign up'
    }
  },
})

export default async function someAction(prev: unknown, formData: FormData) {
  return await serverValidate(formData)
}

This solves the issue where server code was leaking to the client, while retaining the typing and validator adapter benefits from other libraries.

Fixes #710

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