Add extent to Plot.layout configuration

[mwaskom]

This PR adds the extent parameter to Plot.layout.

While part of incremental progress towards full fine-grained layout customization through Plot methods, this update also helps ease some temporary pains. Namely, it may be useful for keeping the legend "outside" of the plot when using Plot.show. Recall that the legend is set to be interior to the figure boundary when Plot.show is called — hence, when pyplot is used to display the plot — to avoid extending past the boundaries of the pyplot window:

(
    so.Plot(tips, x="total_bill", y="tip")
    .add(so.Dot(), label="Some dots")
    .show()
)

Setting the right boundary of the extent to <1 leaves room for the legend:

(
    so.Plot(tips, x="total_bill", y="tip")
    .add(so.Dot(), label="Some dots")
    .layout(extent=(0, 0, .8, 1))
    .show()
)

Getting the position right may require a little fiddling based on the width of the legend itself. This isn't a perfect fix for legend positioning in so.Plot, just workaround (as a special case of a generally-useful configuration).

[codecov]

Codecov Report

Merging #3552 (596ad4f) into master (03d703b) will decrease coverage by 0.01%.
The diff coverage is 95.12%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3552      +/-   ##
==========================================
- Coverage   98.52%   98.52%   -0.01%     
==========================================
  Files          75       75              
  Lines       24668    24701      +33     
==========================================
+ Hits        24305    24336      +31     
- Misses        363      365       +2     

Files	Coverage Δ
seaborn/_compat.py	59.74% <100.00%> (+4.02%)	⬆️
seaborn/_core/subplots.py	100.00% <ø> (ø)
tests/_core/test_plot.py	98.83% <100.00%> (+0.01%)	⬆️
seaborn/_core/plot.py	99.15% <86.66%> (-0.24%)	⬇️