allow find command to look in specified columns only

[fdncred]

Description

This PR allows the find command to search in specific columns using --columns [col1 col2 col3]. This is really meant to help with the help command in the std.nu.

There are a few more things I want to look at so this is a draft for now.

• add example
• look at regex part

User-Facing Changes

Tests + Formatting

After Submitting

[codecov]

Codecov Report

Merging #8937 (e904952) into main (29256b1) will increase coverage by 0.36%.
The diff coverage is 92.59%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8937      +/-   ##
==========================================
+ Coverage   68.18%   68.55%   +0.36%     
==========================================
  Files         636      636              
  Lines      101704   101755      +51     
==========================================
+ Hits        69350    69754     +404     
+ Misses      32354    32001     -353     

Impacted Files	Coverage Δ
crates/nu-command/src/filters/find.rs	79.02% <92.59%> (+1.99%)	⬆️

... and 10 files with indirect coverage changes