Handle get0() in setDT() to preserve .internal.selfref

[venom1204]

closes #6864

hi in this pr I

• Extend setDT() to handle get0() calls (like it does for get()).
• Fix: added get0 to name %iscall% c("get", "get0").

hi @MichaelChirico can you please review this when you have time.

thank you

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.69%. Comparing base (6a6147e) to head (27aad20).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7101   +/-   ##
=======================================
  Coverage   98.69%   98.69%           
=======================================
  Files          79       79           
  Lines       14680    14680           
=======================================
  Hits        14489    14489           
  Misses        191      191           

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[github-actions]

• HEAD=issue__6864 stopped early for DT[by,verbose=TRUE] improved in #6296
  

Generated via commit 27aad20

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

Task	Duration
R setup and installing dependencies	2 minutes and 48 seconds
Installing different package versions	38 seconds
Running and plotting the test cases	2 minutes and 25 seconds

[MichaelChirico]

Thanks! Please add a test and NEWS item

[venom1204]

done