The sub-fields that a user can navigate to from a FieldTree<TModel>.
API
type Subfields<TModel> = {
readonly [K in keyof TModel as TModel[K] extends Function ? never : K]: MaybeFieldTree<
TModel[K],
string
>;
} & {
[Symbol.iterator](): Iterator<[string, MaybeFieldTree<TModel[keyof TModel], string>]>;
}