Rename `Timer::paused` to `Timer::is_paused` and `Timer::finished` to `Timer::is_finished`

[mgi388]

#15962 aligned public apis of Time, Timer and Stopwatch.

Stopwatch::paused was renamed to Stopwatch::is_paused.

But Timer::paused was left as is for some reason (couldn't find why), and also even though it calls Stopwatch::is_paused underneath:

bevy/crates/bevy_time/src/timer.rs

Lines 326 to 328 in 4051465

	pub fn paused(&self) -> bool {
	self.stopwatch.is_paused()
	}

1. So I think we should rename Timer::paused to Timer::is_paused, leaving Timer::paused throughout 0.17, and removing it in 0.18.
2. Similarly for Timer::finished -> Timer::is_finished.
3. I note there is Timer::just_finished. I honestly don't know about that one. I think Timer::is_just_finished probably works and follows the rule, but I could also see Timer::just_finished continuing to make sense with just_ being its own category of predicate naming similar to names like has_ or must_.

I've left the "ready for implementation" tag off until some bike shedding can happen on item 3.