Geom.polygon and Geom.ribbon update

[Mattriks]

• I've updated the documentation to reflect these changes
• I've added an entry to NEWS.md
• I've added and/or updated the unit tests
• I've run the regression tests
• I've built the docs and confirmed these changes don't cause new errors

This PR:

• Support one-length aesthetics for Geom.polygon and Geom.ribbon

Example

x, y = [1, 2, 3, 4, 5], [1.0, 2.0, NaN, 4.0, 5.0]
Dps = reduce(vcat, [DataFrame(x=[1, 5, 5, 1].+d, y=[14, 14, 10, 10].-d, id=d+1) for d in 0:9])

p1 = plot(x=x, y=y, ymin=y.-0.5, ymax=y.+0.5, Geom.line, Geom.point, linestyle=[:dash],
    layer(Geom.ribbon, alpha=[0.2], color=[colorant"gold"]), Theme(lowlight_color=identity)
)
p2 = plot(Dps, x=:x, y=:y, color=:id, alpha=[0.3], linestyle=[:dash],
    Geom.polygon(fill=true), Scale.color_discrete,
    Theme(line_width=2pt, lowlight_color=identity, discrete_highlight_color=identity)
)
hstack(p1, p2))

Potential change:

• The default for lowlight_color could be changed to Theme(lowlight_color=identity), in favor of doing alpha=[a].

Note Lowlight_color may still be useful in some situations (e.g. see Stat.smooth). I'd like feedback on this potential change.

[codecov-io]

Codecov Report

Merging #1511 (be60590) into master (7144517) will decrease coverage by 0.02%.
The diff coverage is 98.03%.

@@            Coverage Diff             @@
##           master    #1511      +/-   ##
==========================================
- Coverage   89.72%   89.70%   -0.03%     
==========================================
  Files          39       39              
  Lines        4625     4613      -12     
==========================================
- Hits         4150     4138      -12     
  Misses        475      475              

Impacted Files	Coverage Δ
src/geometry.jl	77.77% <ø> (ø)
src/theme.jl	71.23% <0.00%> (ø)
src/geom/polygon.jl	100.00% <100.00%> (ø)
src/geom/ribbon.jl	100.00% <100.00%> (ø)
src/misc.jl	64.59% <100.00%> (-3.39%)	⬇️

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