Add slot (ReactNode prop) support

[effulgentsia]

Currently Component definitions can include props with a Zod schema and hasChildren for React's children prop. React also supports props named something other than children that, like children, are also of type ReactNode: what other frameworks call slots.

It would be great to be able to define such components in json-render. Perhaps adding a slots key to the Component definition, like:

{
  props: z.object({...}),  // Zod schema for props
  hasChildren?: boolean,    // Can it have children?
  slots?: [
    {name: 'slot1', description: 'Help AI understand when to use it'},
    {name: 'slot2', description: 'Help AI understand when to use it'},
  ],
  description?: string,     // Help AI understand when to use it
}