add List#scanLeftNel and List#scanRightNel

[enzief]

.scanLeft result is always non-empty.

[codecov-io]

Codecov Report

Merging #3239 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3239      +/-   ##
==========================================
+ Coverage   93.05%   93.05%   +<.01%     
==========================================
  Files         376      376              
  Lines        7413     7415       +2     
  Branches      201      201              
==========================================
+ Hits         6898     6900       +2     
  Misses        515      515

Flag	Coverage Δ
#scala_version_212	93.38% <100%> (ø)	⬆️
#scala_version_213	92.83% <100%> (ø)	⬆️

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

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[LukaJCB]

Hey @enzief, thanks for the contribution, this seems pretty useful! Do you mind adding a test or two? Maybe a doctest? :) Much appreciated!

[rossabaker]

I think a .scanRightNel would also be possible, if you're feeling ambitious. 😄

[enzief]

@LukaJCB @rossabaker another look please.

[LukaJCB]

Looks good, thank you!

[rossabaker]

👍 Thanks.