feat: support trailing commas everywhere

[lgaborini]

Hello!
Super quick PR that should fix #282 in most layers.

Tested so far:

• geom_*
• scale_*
• facet_*
• annotate_*
• label_*
• element_*
• guide_*
• call to girafe()
• call to girafe_options()

I'm getting this warning in the tests but it seems unrelated:

Warning message:
In vp$just : partial match of 'just' to 'justification'

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.04%. Comparing base (550fe25) to head (5b96a99).

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #287   +/-   ##
=======================================
  Coverage   94.04%   94.04%           
=======================================
  Files         106      106           
  Lines        2704     2704           
=======================================
  Hits         2543     2543           
  Misses        161      161           

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[davidgohel]

Thank you @lgaborini, much appriciated