provide data_only mode for wb_load()

[JanMarvin]

This allows reading a striped down version of wbWorkbook with only sheet_data. Thought that this would speed read_xlsx a bit up, but could not measure any difference

[codecov-commenter]

Codecov Report

Merging #181 (2a2f7f6) into main (ddb8ea2) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #181      +/-   ##
==========================================
+ Coverage   78.40%   78.44%   +0.04%     
==========================================
  Files          35       35              
  Lines        7406     7406              
==========================================
+ Hits         5807     5810       +3     
+ Misses       1599     1596       -3     

Impacted Files	Coverage Δ
R/readWorkbook.R	95.65% <100.00%> (-0.10%)	⬇️
R/wb_functions.R	98.52% <100.00%> (+0.37%)	⬆️
R/wb_load.R	95.08% <100.00%> (+0.35%)	⬆️

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