UnicodePlots: use more extra_kwargs

[t-bltg]

p = plot(extra_kwargs=Dict(:subplot=>(; canvas=:dot)))

[codecov]

Codecov Report

Merging #4080 (4a9fbe5) into master (3e8db45) will increase coverage by 0.01%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master    #4080      +/-   ##
==========================================
+ Coverage   62.21%   62.23%   +0.01%     
==========================================
  Files          29       29              
  Lines        7292     7293       +1     
==========================================
+ Hits         4537     4539       +2     
+ Misses       2755     2754       -1     

Impacted Files	Coverage Δ
src/backends.jl	64.60% <ø> (ø)
src/backends/unicodeplots.jl	53.06% <71.42%> (+0.32%)	⬆️
src/backends/gr.jl	88.36% <0.00%> (+0.08%)	⬆️

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[BeastyBlacksmith]

These should be documented somewhere