Use StringColumn for decimal type by default

[vjik]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	❌

Related to yiisoft/db#1125

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.73%. Comparing base (b15aefd) to head (6dcccdc).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #454   +/-   ##
=========================================
  Coverage     98.73%   98.73%           
  Complexity      259      259           
=========================================
  Files            24       24           
  Lines           791      792    +1     
=========================================
+ Hits            781      782    +1     
  Misses           10       10           

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