Add Plot.label method

[mwaskom]

Another specific configuration method. Like Plot.limit, also subject to changes before release on the name (and on whether it should be a distinct function) but status quo bias is strong.

Accepts a string, a function, or None:

(
    so.Plot(tips, "day", "total_bill", color="sex")
    .add(so.Bar(), so.Agg(), so.Dodge())
    .label(x=str.capitalize, y="Total bill ($)", color=None)
)

This API will entail a separation between the text of the labels and any other configuration of them, and I am not sure where other configuration should live as of the moment.

[codecov]

Codecov Report

Merging #2902 (932b8c4) into master (05066e0) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2902   +/-   ##
=======================================
  Coverage   98.20%   98.20%           
=======================================
  Files          69       69           
  Lines       22625    22664   +39     
=======================================
+ Hits        22219    22258   +39     
  Misses        406      406           

Impacted Files	Coverage Δ
seaborn/_core/plot.py	94.93% <100.00%> (+0.13%)	⬆️
tests/_core/test_plot.py	98.45% <100.00%> (+0.02%)	⬆️