Highlighting distinct in sql

[akashdotsrivastava]

Issue #, if available:
Closes #4398

Description of changes:
Added distinct to the list of keywords in SqlHighlightRules in mode-sql

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[codecov]

Codecov Report

Merging #4399 (b2d3a35) into master (8a13579) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master    #4399   +/-   ##
=======================================
  Coverage   70.51%   70.51%           
=======================================
  Files         541      541           
  Lines       54257    54257           
  Branches    10202    10202           
=======================================
  Hits        38262    38262           
  Misses      15995    15995           

Flag	Coverage Δ
unittests	70.51% <0.00%> (ø)

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Impacted Files	Coverage Δ
lib/ace/mode/sql_highlight_rules.js	100.00% <ø> (ø)
lib/ace/virtual_renderer.js	79.06% <0.00%> (ø)

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