fix: Added es6-shim library to fix old browsers

[andrewnester]

Issue #, if available:

Description of changes:
Brought back from c9 branch

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[codecov]

Codecov Report

Merging #4720 (54e845c) into master (bf2c7e2) will decrease coverage by 0.07%.
The diff coverage is 16.86%.

@@            Coverage Diff             @@
##           master    #4720      +/-   ##
==========================================
- Coverage   71.19%   71.11%   -0.08%     
==========================================
  Files         553      554       +1     
  Lines       55598    55678      +80     
  Branches    10392    10415      +23     
==========================================
+ Hits        39584    39597      +13     
- Misses      16014    16081      +67     

Flag	Coverage Δ
unittests	71.11% <16.86%> (-0.08%)	⬇️

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Impacted Files	Coverage Δ
lib/ace/lib/es6-shim.js	15.85% <15.85%> (ø)
lib/ace/lib/fixoldbrowsers.js	100.00% <100.00%> (+50.00%)	⬆️

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