Add Perc stat for computing percentiles

[mwaskom]

(
    so.Plot(tips, "day", "total_bill")
    .add(so.Dot(), so.Perc(11))
    .add(so.Range(linewidth=3), so.Perc([10, 90]), so.Shift(x=.1))
)

[codecov]

Codecov Report

Merging #3063 (1614c6e) into master (c412ddf) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3063   +/-   ##
=======================================
  Coverage   98.38%   98.38%           
=======================================
  Files          72       74    +2     
  Lines       23537    23632   +95     
=======================================
+ Hits        23156    23251   +95     
  Misses        381      381           

Impacted Files	Coverage Δ
seaborn/_stats/aggregation.py	97.50% <ø> (ø)
seaborn/_stats/order.py	100.00% <100.00%> (ø)
seaborn/objects.py	100.00% <100.00%> (ø)
tests/_stats/test_order.py	100.00% <100.00%> (ø)