Add unit.release() method

[olistic]

Move release logic (unbinding and scoring) from the rescue ability to the core.

[codecov-io]

Codecov Report

Merging #140 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #140      +/-   ##
=========================================
+ Coverage   91.08%   91.1%   +0.01%     
=========================================
  Files          81      81              
  Lines        1043    1045       +2     
  Branches      167     167              
=========================================
+ Hits          950     952       +2     
  Misses         77      77              
  Partials       16      16

Impacted Files	Coverage Δ
packages/warriorjs-abilities/src/rescue.js	100% <100%> (ø)	⬆️
packages/warriorjs-core/src/Unit.js	100% <100%> (ø)	⬆️

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