Add Dash mark

[mwaskom]

This mark doesn't have a direct analogue in the classic interface but it's kind of like a category-specific rug plot:

(
    so.Plot(penguins, "species", "body_mass_g", color="sex")
    .add(so.Dash())
)

One useful application is to denote the central tendency behind a strip or swam plot:

(
    so.Plot(penguins, "species", "body_mass_g", color="sex")
    .add(so.Dots(), so.Dodge(), so.Jitter())
    .add(so.Dash(), so.Agg(), so.Dodge())
)

[codecov]

Codecov Report

Merging #3074 (9f4f244) into master (54cab15) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3074   +/-   ##
=======================================
  Coverage   98.38%   98.39%           
=======================================
  Files          74       74           
  Lines       23648    23715   +67     
=======================================
+ Hits        23267    23334   +67     
  Misses        381      381           

Impacted Files	Coverage Δ
seaborn/_marks/line.py	100.00% <100.00%> (ø)
seaborn/objects.py	100.00% <100.00%> (ø)
tests/_marks/test_line.py	100.00% <100.00%> (ø)