テンプレートリテラル型(template literal types)とは何ですか?

type Color = "red" | "blue";
type Shade = "light" | "dark";
type Variant = `${Shade}-${Color}`;
// "light-red" | "light-blue" | "dark-red" | "dark-blue" — all combinations

type Entity = "user" | "post";
type Event = `${Entity}:${"created" | "deleted"}`;
// "user:created" | "user:deleted" | "post:created" | "post:deleted"

function on(event: Event, cb: () => void) {}
on("user:created", () => {}); // ✅
on("user:updated", () => {}); // ❌ not a valid event

type Getters<T> = {
  [K in keyof T & string as `get${Capitalize<K>}`]: () => T[K];
};
// { name: string } → { getName: () => string }