Convert Selection to es6 class

[mkslanc]

Issue #, if available:

Description of changes:

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[codecov]

Codecov Report

Patch coverage: 97.59% and project coverage change: -0.01% ⚠️

Comparison is base (d41b7c7) 87.25% compared to head (9110b45) 87.25%.
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5289      +/-   ##
==========================================
- Coverage   87.25%   87.25%   -0.01%     
==========================================
  Files         567      567              
  Lines       45338    45368      +30     
  Branches     6934     6930       -4     
==========================================
+ Hits        39561    39587      +26     
- Misses       5777     5781       +4     

Flag	Coverage Δ
unittests	87.25% <97.59%> (-0.01%)	⬇️

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Files Changed	Coverage Δ
src/selection.js	92.68% <97.59%> (-0.49%)	⬇️

... and 5 files with indirect coverage changes

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