- Add `ForegroundExecutor::spawn_when_idle(timeout, future)`, which
schedules `future` to be polled on the main thread when the main thread
is idle, but tries to do so no later than `timeout` after the current
instant (with the same `timeout` being applied to subsequent polls of
the same future)
- Add `Platform::dispatch_on_main_thread_when_idle(runnable, timeout)`
as the basis for `spawn_when_idle`, with a default implementation that
ignores the timeout and delegates to `dispatch_on_main_thread(runnable,
Priority::Low)`
- Implement `dispatch_on_main_thread_when_idle` for the web platform
using
[`requestIdleCallback`](https://developer.mozilla.org/en-US/docs/Web/API/Window/requestIdleCallback)

Release Notes:
- N/A