New nzchar_linter

[MichaelChirico]

Part of #884

Some notes:

• We could easily make the behavior we chose to skip linting if (x == "") optional
• This linter may be somewhat controversial because of the difference in handling NA_character_. I really find the mismatch of defaults in R confusing here. We could consider a weaker version of this linter that only lints nchar(x, keepNA = FALSE)
• No lints on {lintr}

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7970260) 99.45% compared to head (fdf83e4) 99.45%.

❗ Current head fdf83e4 differs from pull request most recent head dc1e515. Consider uploading reports for the commit dc1e515 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2275   +/-   ##
=======================================
  Coverage   99.45%   99.45%           
=======================================
  Files         116      117    +1     
  Lines        5286     5341   +55     
=======================================
+ Hits         5257     5312   +55     
  Misses         29       29           

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[AshesITR]

Missing metadata tests.

[IndrajeetPatil]

@MichaelChirico This should be ready for a merge after the merge conflict is resolved.