Throttle Implementation

Implement a throttle function that takes a callback function and a delay in milliseconds, and returns a new "throttled" version of that callback. The throttled function may be invoked repeatedly, but the original callback must be executed at most once per delay period. Specifically:

• The very first call to the throttled function executes the callback immediately.
• Subsequent calls that occur within the delay period are ignored (they do NOT execute the callback).
• After the full delay has elapsed since the last real execution, the next call may execute the callback again, restarting the cycle.
• The returned throttled function must preserve the original function’s this context and all supplied arguments.
• Provide a .cancel() method on the returned throttled function that cancels any pending scheduled execution and resets internal state so the next call will again be treated as the "first" call.

This is the classic "leading-edge" throttle with no trailing call.