[FEATURE REQUEST] Add frametimes relative standard deviation (Coefficient of Variation) measurement

Frametime stability and smoothness is probably better measured with standard deviation based metrics (https://en.wikipedia.org/wiki/Coefficient_of_variation) more than just percentiles. While percentiles are good for capturing stutters and some instability, they tend to be completely blind to the chaos near the average. Standard deviation divided by the mean could be a great addition as it's standard statistical tool that reflect all variance in all samples, with skew toward outliers.

One problem is this measure is not common even in advanced tools like presentmon, so people may not be familiar with its implication, but we can present it as 'stability' or other common terms.

If this is within the scope of mangohud, i'm willing to work on it.