Improve MySQL highlighting mode

[rozhnev]

Improve MySQL highlighting mode:

• separate dataTypes highlighting mode
• added timestamp dataTypes
• 'by' keyword moved from builtins to mySqlKeywords
• date|time|timestamp moved from constants to builtins
  Description of changes:

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[codecov]

Codecov Report

Base: 86.13% // Head: 86.14% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (6169aef) compared to base (95e9664).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5050   +/-   ##
=======================================
  Coverage   86.13%   86.14%           
=======================================
  Files         546      548    +2     
  Lines       41677    41703   +26     
  Branches     6563     6563           
=======================================
+ Hits        35900    35926   +26     
  Misses       5777     5777           

Flag	Coverage Δ
unittests	86.14% <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/mysql_highlight_rules.js	100.00% <100.00%> (ø)
src/mode/plsql.js	100.00% <100.00%> (ø)
src/mode/plsql_highlight_rules.js	100.00% <100.00%> (ø)

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