updateTimeScale method
Updates timeScale by applying fn to its current value.
A convenience wrapper around setTimeScale for relative changes, so you don't need to read timeScale yourself first. Goes through setTimeScale, so the usual negative-value clamping still applies.
Example:
app.time.updateTimeScale((ts) => ts - 0.1); // slow down by 0.1
Implementation
void updateTimeScale(double Function(double timeScale) fn)
=> setTimeScale(fn(_timeScale));