Enhance cols_*() functions

[jrycw]

This PR aims to enhance the cols_*() functions by:

• Abstracting the validation of sel_cols into a separate function and adding related tests.
• Suggesting renaming vars to col_vars in the function. While vars might be acceptable in the function signature, it’s better to avoid using vars as a temporary variable name if possible.
• Updating the syntax for constructing the DataFrame in the cols_*() test functions.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.06%. Comparing base (e3649d2) to head (642cf2e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #366      +/-   ##
==========================================
+ Coverage   85.90%   86.06%   +0.16%     
==========================================
  Files          41       41              
  Lines        4328     4321       -7     
==========================================
+ Hits         3718     3719       +1     
+ Misses        610      602       -8     

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[machow]

This looks great, thanks!