75 add image plot

[jmbarbone]

related to #75

works on wb_add_plot() and wb_add_image(). These ones took a bit more work and now wbWorkbook$add_plot() is now a more direct wrapper for wbWorkbook$add_image(). The testing is a little weird though and probably requires a bit more work for expect_wrapper() to be more certain.

[JanMarvin]

Codecov Report

Merging #145 (a14b9b1) into main (568d550) will increase coverage by 1.04%.
The diff coverage is 36.36%.

@@            Coverage Diff             @@
##             main     #145      +/-   ##
==========================================
+ Coverage   71.77%   72.81%   +1.04%     
==========================================
  Files          37       37              
  Lines        7220     7250      +30     
==========================================
+ Hits         5182     5279      +97     
+ Misses       2038     1971      -67     

Impacted Files	Coverage Δ
R/class-workbook.R	68.91% <30.15%> (+1.58%)	⬆️
R/class-workbook-wrappers.R	50.19% <50.00%> (+4.40%)	⬆️
R/expect-wrapper.R	69.86% <100.00%> (+0.41%)	⬆️
R/baseXML.R	84.69% <0.00%> (+7.11%)	⬆️

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[JanMarvin]

Note: the plot tests only complete in interactive R sessions, such as in RStudio.
Otherwise rebased and merged this PR as well!