Change property Schema::$typeMap to constant Schema::TYPE_MAP

[Tigrov]

The change reduces size of cache.

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	❌
Fixed issues	-

[what-the-diff]

PR Summary

• Introduction of Enhanced Consistency
  A new constant named TYPE_MAP has been introduced in the Schema class, enhancing predictability and consistency. An existing property typeMap has now been restricted for private access only, safeguarding it from unintended modifications.

• Refining References
  The existing references to the typeMap property have been updated. They now utilize the constant TYPE_MAP, reducing the risk of errors by guaranteeing stable values.

• Improved Method Implementation
  The loadColumnSchema method has been enhanced. It employs the constant TYPE_MAP when establishing the column type, ensuring accurate type mapping and improving the reliability of schema loading operation.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (907f8e5) 98.48% compared to head (1ca28a8) 98.48%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #286   +/-   ##
=========================================
  Coverage     98.48%   98.48%           
  Complexity      205      205           
=========================================
  Files            15       15           
  Lines           596      596           
=========================================
  Hits            587      587           
  Misses            9        9           

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