enable use of by=.I

[ben-schwen]

Closes #1732

edit: Current implementation supports:

by = .I
by = .(.I)
by = list(.I)
by = c(.I)

[codecov]

Codecov Report

Merging #5235 (e136746) into master (75b5d00) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #5235   +/-   ##
=======================================
  Coverage   99.50%   99.50%           
=======================================
  Files          77       77           
  Lines       14655    14660    +5     
=======================================
+ Hits        14583    14588    +5     
  Misses         72       72           

Impacted Files	Coverage Δ
R/data.table.R	99.89% <100.00%> (+<0.01%)	⬆️

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[MichaelChirico]

I think users will expect by=.(.I) to work as well...

it's fine if its hard to implement that that we skip it for now, but we should clarify that a bit more in the NEWS/docs. as written it will come as a surprise to users.

[ben-schwen]

I think users will expect by=.(.I) to work as well...

Good point. Changed it to support all of the following:

by = .I
by = .(.I)
by = list(.I)
by = c(.I)