Add highlight mode for Apache JEXL

[mkslanc]

Issue #, if available: #4648

Description of changes:

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[codecov]

Codecov Report

Base: 86.08% // Head: 86.09% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (098a460) compared to base (0c49167).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4979   +/-   ##
=======================================
  Coverage   86.08%   86.09%           
=======================================
  Files         544      546    +2     
  Lines       41511    41538   +27     
  Branches     6547     6547           
=======================================
+ Hits        35735    35762   +27     
  Misses       5776     5776           

Flag	Coverage Δ
unittests	86.09% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/mode/jexl.js	100.00% <100.00%> (ø)
src/mode/jexl_highlight_rules.js	100.00% <100.00%> (ø)

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[andrewnester]

Thank you!