Adds optional column count display to print.data.table()

[venom1204]

closes #6663.
in this pr I

• Added datatable.show.ncols option (default FALSE).
• Added show.ncols support .
• Added header output for total column count.
• Integrated column count with trunc.cols=TRUE output.

hi @tdhock @joshhwuu can you please have a look when you get time.
thanks

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.85%. Comparing base (d65e46f) to head (43dddf2).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7791   +/-   ##
=======================================
  Coverage   98.85%   98.85%           
=======================================
  Files          87       87           
  Lines       17128    17139   +11     
=======================================
+ Hits        16932    16943   +11     
  Misses        196      196           

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[github-actions]

• HEAD=issue6663 much slower for transform improved in #5493
• HEAD=issue6663 slower P<0.001 for DT[by] max regression fixed in #7480
• HEAD=issue6663 faster P<0.001 for memrecycle regression fixed in #5463
• HEAD=issue6663 faster P<0.001 for fwrite refactored in #6393
  

Generated via commit 43dddf2

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	2 minutes and 42 seconds
Installing different package versions	24 seconds
Running and plotting the test cases	5 minutes and 31 seconds

[tdhock]

looks good, please add NEWS

[tdhock]

please remove empty lines