FR: show exact p-value instead of "P<0.001" for small p-values in `stat_correlation()`, `stat_poly_eq()` and `stat_ma_eq()` and `stat_multcomp()`

[wbvguo]

Dear ggpmisc developer,

I was wondering if we could provide a option to show the exact p-values in scientific form for the very small p-values. For example when p=0.000000001, it's great to see p = 1e-9

Currently the ggpmisc only shows the label P < 0.001. Users can change p.digits to indicate how many digits to show, but it doesn't look good if there are too many 0s

Thanks!