Add ion highlight mode

[jpschorr]

Add syntax mode for Amazon Ion (https://amzn.github.io/ion-docs/) textual notation (https://amzn.github.io/ion-docs/docs/spec.html)

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[codecov]

Codecov Report

Merging #4629 (1d5e9c0) into master (8a991cc) will increase coverage by 0.01%.
The diff coverage is 84.09%.

@@            Coverage Diff             @@
##           master    #4629      +/-   ##
==========================================
+ Coverage   71.09%   71.10%   +0.01%     
==========================================
  Files         547      549       +2     
  Lines       55429    55473      +44     
  Branches    10391    10393       +2     
==========================================
+ Hits        39405    39442      +37     
- Misses      16024    16031       +7     

Flag	Coverage Δ
unittests	71.10% <84.09%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
lib/ace/mode/ion.js	75.86% <75.86%> (ø)
lib/ace/mode/ion_highlight_rules.js	100.00% <100.00%> (ø)

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