feat: Generic `Spec` type support

[vanduyquang]

Hi, it seems like we don't support exporting a generic Spec type, currently Spec looks like this:

interface Spec {
    /** Root element key */
    root: string;
    /** Flat map of elements by key */
    elements: Record<string, UIElement>;
    /** Optional initial state to seed the state model.
     *  Components using statePath will read from / write to this state. */
    state?: Record<string, unknown>;
}

However, UIElement is a fully generic type:

interface UIElement<T extends string = string, P = Record<string, unknown>> {
    /** Component type from the catalog */
    type: T;
    /** Component props */
    props: P;
// ...

I wonder if we could add support for a Spec<T, P> type as well. For context, I want to pass Spec as a type to a React Hook Form form, but the type of elements should be limited to my own catalog only, and I have no way to that now without redefining Spec myself.

Thank you.