Add Est stat and Interval mark to show error bars

[mwaskom]

This PR adds a new mark and a new stat that together will add error bars to aggregations:

(
    so.Plot(diamonds, x="carat", y="clarity")
    .add(so.Interval(), so.Est("mean", "sd"))
    .add(so.Dot(), so.Agg())
)

The Est stay has the same basic API for error bar selection as we'll be rolling out for the functional interface in 0.12 (i.e., ci/se/pi/sd, with an optional scale/level parameter.

Note that is currently necessary to add two layers to get a dot + interval; see #2911 for some ideas there

This PR also addresses some wrinkles relating to matplotlib capstyles in both the interval mark and existing line marks.

[mwaskom]

Here's something that needs to be addressed:

(
    so.Plot(penguins, x="bill_length_mm", y="species", color="sex")
    .add(so.Interval(), so.Est(), so.Dodge())
    .add(so.Dot(), so.Agg(), so.Dodge())
)

This is because Interval and Dot end up with different grouping_props; it can be worked around:

(
    so.Plot(penguins, x="bill_length_mm", y="species", color="sex")
    .add(so.Interval(), so.Est(), so.Dodge(by=["color"]))
    .add(so.Dot(), so.Agg(), so.Dodge())
)

But this needs a cleaner solution (cf #2894)

[codecov]

Codecov Report

Merging #2912 (edb5c3c) into master (b726628) will increase coverage by 0.01%.
The diff coverage is 99.43%.

@@            Coverage Diff             @@
##           master    #2912      +/-   ##
==========================================
+ Coverage   98.20%   98.22%   +0.01%     
==========================================
  Files          69       69              
  Lines       22666    22822     +156     
==========================================
+ Hits        22260    22416     +156     
  Misses        406      406              

Impacted Files	Coverage Δ
seaborn/_stats/histograms.py	98.82% <ø> (ø)
seaborn/_core/scales.py	89.36% <75.00%> (-0.17%)	⬇️
seaborn/_core/plot.py	94.97% <100.00%> (+0.02%)	⬆️
seaborn/_marks/bars.py	100.00% <100.00%> (ø)
seaborn/_marks/lines.py	100.00% <100.00%> (ø)
seaborn/_stats/aggregation.py	97.50% <100.00%> (+5.50%)	⬆️
seaborn/objects.py	100.00% <100.00%> (ø)
tests/_marks/test_lines.py	100.00% <100.00%> (ø)
tests/_stats/test_aggregation.py	100.00% <100.00%> (ø)