Remove second associateBy parameter, replace with associateByTransform

[passsy]

Remove second associateBy parameter, replace with associateByTransform with two parameters

associateBy is much more often used with a single parameter. This wasn’t possible until now because the second lambda value type V couldn’t only be known when the second lambda was provided

[codecov]

Codecov Report

Merging #17 into master will increase coverage by 0.75%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
+ Coverage   56.66%   57.42%   +0.75%     
==========================================
  Files          24       24              
  Lines        1170     1172       +2     
==========================================
+ Hits          663      673      +10     
+ Misses        507      499       -8

Impacted Files	Coverage Δ
lib/src/extension/iterable_extension_mixin.dart	49.69% <100%> (+1.83%)	⬆️

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