if_not_else_linter() for "double negation" in if statements

[MichaelChirico]

Part of #884.

Internally, we use the default exceptions=, but I'd be open to setting that to character() by default, or extending the current list.

[codecov-commenter]

Codecov Report

Merging #2071 (81b518f) into main (15a22fe) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 81b518f differs from pull request most recent head e8f755d. Consider uploading reports for the commit e8f755d to get more accurate results

@@           Coverage Diff           @@
##             main    #2071   +/-   ##
=======================================
  Coverage   99.65%   99.65%           
=======================================
  Files         117      118    +1     
  Lines        5164     5201   +37     
=======================================
+ Hits         5146     5183   +37     
  Misses         18       18           

Files Changed	Coverage Δ
R/if_not_else_linter.R	100.00% <100.00%> (ø)
R/xp_utils.R	100.00% <100.00%> (ø)