Rate limiter with sliding window

Implement a sliding-window rate limiter in TypeScript, exported as a class RateLimiter from rate_limiter.ts. Constructor: (limit: number, windowMs: number, now: () => number = Date.now). Method check(key: string): {allowed: boolean; remaining: number; retryAfterMs: number}. Use a true sliding window (not fixed buckets): a request is allowed if fewer than limit requests for that key occurred in the last windowMs. retryAfterMs is 0 when allowed, otherwise the time until the oldest in-window request expires. Memory must not grow unbounded: expose prune(): number that drops expired entries and returns how many keys were removed entirely. No dependencies. Include JSDoc. Return only the file in one code block.