Add `.just_started` (or equivalent) to `Timer`

## What problem does this solve or what need does it fill?

Hypothetical:

I have an event happen, such as some collision event.
I have _N_ timers, which I then query and `.reset()`.

I might have some systems which depend on some of these timers.
I'd want to know when these timers start, such that I can react to them (e.g. start sound effect, add buff to creature, etc.).

`Timer` already has `.just_finished`, which is nice in order to reverse/stop the added effect (or buff, ...).

## What solution would you like?

Add `.just_started` (or similar name) to `Timer`.

## What alternative(s) have you considered?

Just react to each timer in the location where `.reset()` is used.
This gets uglier the more timers there are, and it couples the functionality of the `Timer` to the thing that depends on it.