fix(chips): add chip leading icon class management

[moog16]

fixes #270

[codecov-io]

Codecov Report

Merging #281 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #281      +/-   ##
==========================================
+ Coverage   97.41%   97.43%   +0.02%     
==========================================
  Files          33       33              
  Lines        1275     1289      +14     
  Branches      125      126       +1     
==========================================
+ Hits         1242     1256      +14     
  Misses         33       33

Impacted Files	Coverage Δ
packages/chips/Chip.js	97.29% <100%> (+0.52%)	⬆️
packages/chips/ChipSet.js	98.59% <100%> (+0.04%)	⬆️

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[bonniezhou]

The leading icon is missing some transitions. I think we'll have to use a ref similar to #273, since the leading icon transition depends on requestAnimationFrame