Add Access-Control header for CORS

[nicolaisueper]

This should fix #570 👀

Regards
Nico

[codecov-io]

Codecov Report

Merging #713 into master will decrease coverage by 0.63%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #713      +/-   ##
==========================================
- Coverage   89.57%   88.94%   -0.64%     
==========================================
  Files          62       62              
  Lines        1986     1990       +4     
==========================================
- Hits         1779     1770       -9     
- Misses        207      220      +13

Impacted Files	Coverage Δ
src/Server.js	91.66% <100%> (+0.75%)	⬆️
src/worker.js	90.9% <0%> (-9.1%)	⬇️
src/assets/CSSAsset.js	82.35% <0%> (-7.36%)	⬇️
src/utils/installPackage.js	90.62% <0%> (-6.25%)	⬇️
src/Logger.js	43.54% <0%> (-4.84%)	⬇️
src/WorkerFarm.js	91.22% <0%> (-1.76%)	⬇️

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