Add removeFirstOrNull

[Dev-dfm]

fixes #159 removeFirstOrNull

[codecov]

Codecov Report

Merging #173 (35337cd) into master (b29dc21) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 35337cd differs from pull request most recent head ecd5f69. Consider uploading reports for the commit ecd5f69 to get more accurate results

@@           Coverage Diff           @@
##           master     #173   +/-   ##
=======================================
  Coverage   99.07%   99.07%           
=======================================
  Files          40       40           
  Lines        1843     1847    +4     
=======================================
+ Hits         1826     1830    +4     
  Misses         17       17           

Flag	Coverage Δ
unittests	99.07% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
lib/src/collection/impl/list_mutable.dart	100.00% <ø> (ø)
lib/src/collection/kt_list_mutable.dart	100.00% <100.00%> (ø)

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[passsy]

Please make sure to have a minimum diff. It makes it unnecessary noisey to review unrelated formatting changes.

[passsy]

While checking the kotlin implementation I noticed that removeFirst/removeLast were accidentally defined as members of KtMutableList not as extension. Changed that and added removeLastOrNull as well