feat: slice attribute from next position if index is 0

[LucasXu0]

1. Slice attributes from the next position only if the cursor index is 0.
2. Slice attributes from the previous position if the cursor is not 0.

showcase.mp4

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 71.94%. Comparing base (98356fa) to head (1781087).

Files with missing lines	Patch %	Lines
lib/src/core/document/text_delta.dart	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #911      +/-   ##
==========================================
- Coverage   71.94%   71.94%   -0.01%     
==========================================
  Files         318      318              
  Lines       14980    14984       +4     
==========================================
+ Hits        10778    10780       +2     
- Misses       4202     4204       +2     

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