Autoprint subclass

[MichaelChirico]

Closes #3029.

NB: This is currently based to #6589 given it touches a very similar aspect of print.data.table() (hence cc @aitap too).

This constitutes a small breaking change by restoring behavior of print(DT) and print(DT[, a:=b]) to always print. I don't really see a downside of this -- the other tests in autoprint continue to WAI.

Does anyone see a reason to maintain the old behavior? @jangorecki @tdhock @ben-schwen. It should be feasible to adjust this PR to make the code a tiny bit more complicated while still achieving the desired fix for #3029 -- I just am not seeing the benefit of that (besides back-compatibility on this pretty minor aspect of the package).

Here's the relevant commit that introduced this behavior: 5d95da3 fixing #1122.

It seems like this PR achieves what that PR couldn't (possibly due to an underlying change in R itself), namely to have if (TRUE) DT[, a:=b] and print(DT) both not print.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.61%. Comparing base (2557fa2) to head (5a7882d).

Additional details and impacted files

@@              Coverage Diff               @@
##           knit_print    #6631      +/-   ##
==============================================
- Coverage       98.61%   98.61%   -0.01%     
==============================================
  Files              79       79              
  Lines           14533    14532       -1     
==============================================
- Hits            14332    14331       -1     
  Misses            201      201              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[github-actions]

Generated via commit 5a7882d

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

Task	Duration
R setup and installing dependencies	4 minutes and 37 seconds
Installing different package versions	7 minutes and 39 seconds
Running and plotting the test cases	2 minutes and 15 seconds

[MichaelChirico]

cc @eliocamp @nikosbosse @seabbs @sbfnk in case you want to test out the proposed fix for your use case.