feat(async/unstable): allow setting dynamic timeframe for throttle

[lionel-rowe]

Closes #6796.

If fn returns a promise, lastExecution is now based on the asynchronous end time of the previous call. For example with this function:

async function fn() {
    const start = performance.now()
    while (performance.now() - start < 50) { /* idle  loop */ }
    await new Promise((res) => setTimeout(res, 100))
}

If the throttled function is called once at time 0 and we observe lastExecution at time 999, the current implementation would have it set to ~50, whereas the new implementation has it set to ~150. This will also affect how throttling works with asynchronous functions — the current implementation basically doesn't throttle at all for typical async functions, whereas the new one does.