Unify units of time throughout the project

[mattco98]

Elementa uses multiple units of time in various APIs, when it should really only ever use one. We need to choose between using floats for number of seconds, or Longs for number of milliseconds. This has to be resolved before release of 2.0, as it would be a breaking change.

[FalseHonesty]

What uses millis? I have always intended to use seconds throughout the project, and thought I stuck to that?

[mattco98]

I used milliseconds in the timer API because it mirrors every other API that deals with timers (Kotlin timers, JS's setInterval/setTimeout, etc). I can change it to float for consistency.