feat: deep copy the node

[LucasXu0]

Add a deepCopy function to improve the readability of the original copyWith function.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 72.14%. Comparing base (c6909a7) to head (7a7a8ae).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/src/core/document/node.dart	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #995      +/-   ##
==========================================
- Coverage   72.15%   72.14%   -0.01%     
==========================================
  Files         319      319              
  Lines       15239    15241       +2     
==========================================
  Hits        10996    10996              
- Misses       4243     4245       +2     

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