Differentiate Line/Path and add Lines/Paths alternatives

[mwaskom]

There are two API decisions here that I am not 100% confident about:

• Line vs Path for sorted vs. unsorted, with Path also preserving nulls (showing a break in the path) but Line dropping them.
• Line vs Lines (and equivalent) for using a Line2D artist vs. a LineCollection.

One alternative would have been to have a single Line mark with a sort: bool parameter, and perhaps a collection: bool parameter. I think it makes sense to expose functionality more obviously, and to try and limit mark parameters to mappables? Although, at some point the number of distinct marks would get out of control and it would be hard to find one you are looking for.

The other weird thing about this convention is that many singular-named marks that draw with collections and hence are pretty fast. So the rule becomes something like; when there is a singular/plural pair, it implies that the singular mark is slow (but more fully featured or easier to work with as an artist) but a singular mark itself does not imply slowness? Kind of messy.

[codecov]

Codecov Report

Merging #2822 (b5f3786) into master (7d1c50f) will increase coverage by 0.08%.
The diff coverage is 97.94%.

@@            Coverage Diff             @@
##           master    #2822      +/-   ##
==========================================
+ Coverage   97.96%   98.04%   +0.08%     
==========================================
  Files          67       68       +1     
  Lines       21800    21991     +191     
==========================================
+ Hits        21356    21561     +205     
+ Misses        444      430      -14     

Impacted Files	Coverage Δ
seaborn/tests/_marks/test_bars.py	87.30% <ø> (+6.12%)	⬆️
seaborn/_core/plot.py	94.54% <54.54%> (-0.60%)	⬇️
seaborn/_marks/base.py	97.58% <100.00%> (+0.85%)	⬆️
seaborn/_marks/lines.py	100.00% <100.00%> (ø)
seaborn/objects.py	100.00% <100.00%> (ø)
seaborn/tests/_marks/test_lines.py	100.00% <100.00%> (ø)

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