Convert constants to es module

[irfanhudda]

Associated Issue: #1884

Summary of Changes

• Change constants.js to es module

[codecov]

Codecov Report

Merging #2722 into master will decrease coverage by 0.82%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2722      +/-   ##
==========================================
- Coverage    61.5%   60.68%   -0.83%     
==========================================
  Files          56       55       -1     
  Lines        2190     2144      -46     
  Branches      432      432              
==========================================
- Hits         1347     1301      -46     
  Misses        843      843

Impacted Files	Coverage Δ
src/utils/test-head.js	61.9% <ø> (-1.74%)	⬇️
src/reducers/async-requests.js	92.3% <ø> (-0.55%)	⬇️
src/reducers/ui.js	84.44% <ø> (-0.34%)	⬇️
src/reducers/coverage.js	56.25% <ø> (-2.58%)	⬇️
src/reducers/pause.js	18.66% <ø> (-1.08%)	⬇️

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