Support drop=TRUE in subset.data.table

[MichaelChirico]

Closes #7859

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.01%. Comparing base (accac97) to head (5146eac).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7860   +/-   ##
=======================================
  Coverage   99.01%   99.01%           
=======================================
  Files          88       88           
  Lines       17288    17289    +1     
=======================================
+ Hits        17118    17119    +1     
  Misses        170      170           

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[github-actions]

• HEAD=subset-drop much slower for transform improved in #5493
  

Generated via commit 5146eac

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

Task	Duration
R setup and installing dependencies	6 minutes and 20 seconds
Installing different package versions	12 minutes and 28 seconds
Running and plotting the test cases	5 minutes and 38 seconds