fix: update selection from the delta

[LucasXu0]

In the common case, the cursor will be placed after the composing text. So there's no need to update the selection manually.

However, on Windows and for some IMEs, the cursor will be placed at the front of the composing text. So updating the selection from the deltas is sufficient.

[codecov]

Codecov Report

Attention: Patch coverage is 92.10526% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 74.84%. Comparing base (ffe4b08) to head (b9a7438).

Files	Patch %	Lines
...editor/editor_component/service/ime/text_diff.dart	72.72%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #788      +/-   ##
==========================================
- Coverage   74.86%   74.84%   -0.03%     
==========================================
  Files         300      300              
  Lines       14181    14186       +5     
==========================================
  Hits        10617    10617              
- Misses       3564     3569       +5     

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