Add proportion-based normalization in histplot

[mwaskom]

Follows #2461. This gives the same result as but may be more intuitive for some people.

[codecov]

Codecov Report

Merging #2634 (0e34465) into master (feef114) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2634   +/-   ##
=======================================
  Coverage   97.46%   97.46%           
=======================================
  Files          17       17           
  Lines        6386     6386           
=======================================
  Hits         6224     6224           
  Misses        162      162           

Impacted Files	Coverage Δ
seaborn/_statistics.py	100.00% <100.00%> (ø)

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