Structure selector.
type StructuresField<T extends string = string> = TitledHandle<T> | TitledHandle<T>[];
type MainStructure = StructuresField<"main">;
// Result: {
// handle: "main";
// title: string;
// } | {
// handle: "main";
// title: string;
// }[]
type AllStructures = StructuresField<"main" | "secondary">;
// Result: {
// handle: "main" | "secondary";
// title: string;
// } | {
// handle: "main" | "secondary";
// title: string;
// }[]