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]

Codecov Report

Base: 50.00% // Head: 50.00% // No change to project coverage 👍

Coverage data is based on head (b20bf46) compared to base (4d848de).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #220   +/-   ##
=======================================
  Coverage   50.00%   50.00%           
=======================================
  Files          15       15           
  Lines         164      164           
  Branches       28       28           
=======================================
  Hits           82       82           
  Misses         77       77           
  Partials        5        5           

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[mphilbrick211]

@Mashal-m - when you are ready to have this reviewed, please tag Colin Brash. Thank you!

[Mashal-m]

Ready to review.
@colinbrash

[mphilbrick211]

Hi @colinbrash - friendly ping on this :) Could you please take a look?