Add a `field()` helper for easier schema creation

[atinux]

One example used in Nuxt Studio:

export default defineNuxtSchema({
  appConfig: {
    /**
     * myTheme configuration.
     * @studio-icon carbon:blog
    */
    myTheme: {
      title: field({
        description: 'Website title, used as header default title and meta title.',
        icon: 'material-symbols:title',
        default: 'My Theme',
      }),
      description: field({
        description: 'Your website description, used in meta description and social links.',
        icon: 'material-symbols:description',
        default: 'My agency shiny theme.'
      }),
})

Private repo for now but can help: https://github.com/nuxtlabs/studio-module/blob/dev/src/theme.ts