Separate column type constants

[Tigrov]

Related PR

• Separate column type constants db#877

Q	A
Is bugfix?	❌
New feature?	✔️/❌
Breaks BC?	✔️
Fixed issues	yiisoft/db#854

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.51%. Comparing base (2402d7d) to head (c30bad4).

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #359   +/-   ##
=========================================
  Coverage     99.51%   99.51%           
  Complexity      286      286           
=========================================
  Files            26       26           
  Lines           822      822           
=========================================
  Hits            818      818           
  Misses            4        4           

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