fix: debounce flush should cancel pending invocation

[Yanhu007]

Fixes #256

Problem

debounce.flush() calls the function immediately but does not cancel the pending debounced timer. This causes the function to execute twice — once from flush and once when the timer fires.

const fn = debounce({ delay: 100 }, callback)
fn()           // starts timer
fn.flush()     // calls immediately, but timer still pending!
// 100ms later: callback fires AGAIN

Fix

Clear the timer and reset it to undefined in flush() before calling the function. This matches lodash behavior where flush cancels the delayed invocation.

[vercel]

@Yanhu007 is attempting to deploy a commit to the rayunishinedev's projects Team on Vercel.

A member of the Team first needs to authorize it.