add groupBy to NonEmptyList and groupByNel to List syntax

[julien-truffaut]

No description provided.

[codecov-io]

Codecov Report

Merging #1580 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1580      +/-   ##
==========================================
+ Coverage   92.34%   92.36%   +0.01%     
==========================================
  Files         247      247              
  Lines        3907     3916       +9     
  Branches      136      132       -4     
==========================================
+ Hits         3608     3617       +9     
  Misses        299      299

Impacted Files	Coverage Δ
core/src/main/scala/cats/syntax/list.scala	100% <100%> (ø)	⬆️
core/src/main/scala/cats/data/NonEmptyList.scala	98.94% <100%> (+0.09%)	⬆️

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[kailuowang]

👍 LGTM

[ceedubs]

👍 love it.