build: use shared browserslist configuration

[Mashal-m]

Ticket:
Use shared @edx/browserslist-config

• Removes custom browserslist configuration in favor of using a shared configuration.
• Removes is-es5 check in CI since ES5 was only needed for IE 11 support which is dropped. The supported browsers defined by the shared configuration all natively support ES6.

This change reduces the resultant asset bundle size.

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[codecov-commenter]

Codecov Report

Merging #84 (79e9e66) into master (0130b52) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #84   +/-   ##
=======================================
  Coverage   82.32%   82.32%           
=======================================
  Files          46       46           
  Lines         679      679           
  Branches      132      132           
=======================================
  Hits          559      559           
  Misses        120      120           

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[MaxFrank13]

Thanks for these udpates!