Add method className(), don't break BC third party extensions.

[terabytesoftw]

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

View comment: #19894 (comment)

[what-the-diff]

PR Summary

• Introduction of New Method in Base Class
  A fresh function named className() has been incorporated in our fundamental class file, BaseObject.php. It is tasked with providing the complete name of the class it's applied on.

• New Test for the Base Class Method
  To ensure the new method's functionality and robustness, a test named testClassName() has been created in BaseObjectTest.php. This evaluates the performance of the className() function under various scenarios.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +39.06% 🎉

Comparison is base (6618aa8) 28.12% compared to head (726e641) 67.18%.

Additional details and impacted files

@@              Coverage Diff              @@
##                2.2   #19973       +/-   ##
=============================================
+ Coverage     28.12%   67.18%   +39.06%     
- Complexity    11209    11210        +1     
=============================================
  Files           424      424               
  Lines         35617    35619        +2     
=============================================
+ Hits          10016    23932    +13916     
+ Misses        25601    11687    -13914     

Files Changed	Coverage Δ
framework/base/BaseObject.php	100.00% <100.00%> (+56.09%)	⬆️

... and 203 files with indirect coverage changes

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