[Enhancement] Add move_skew and move_kurt

[jkadbear]

I tried to implement moving window skewness and kurtosis on this project. However, simply applying the technique used in move_std/move_var is not numerically stable due to the third/fourth power needed in skewneww/kurtosis. Are there any methods to reduce the float error?

[OldChi]

I agree. Hope there will be more built-in functions. e.g. move_skew, move_kurt, move_mode. Although I implemented algorithms with best time complexity, the speed is still limited since I'm only writing python.

[kaixiongg]

something like that: https://www.johndcook.com/blog/skewness_kurtosis/