[Feature Proposal] add new StatsAPI function to extract statistics

[babaq]

Currently, only pvalue and confint were proposed and supported by different tests, but sometimes the underlying test statistics were of interest. It would be a more consistent and convenient way to extract statistics by adding new API like stats.

[ararslan]

Duplicate of #79 🙂

[ararslan]

I've finally proposed a solution to that issue, 7 years after opening it: JuliaStats/StatsAPI.jl#24. Feel free to comment there or the original issue if you have further input to provide. Thanks!