refactor: Introduce TaskScope and TaskContext

[achimnol]

This PR adds TaskContext and TaskScope to reuse the core concepts and logics for structured concurrency in Supervisor and other coroutine aggregation utilities like as_completed_safe(), gather_safe(), and race().

[codecov]

Codecov Report

Attention: Patch coverage is 71.80617% with 64 lines in your changes missing coverage. Please review.

Project coverage is 74.00%. Comparing base (75ac9e3) to head (5647a22).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/aiotools/taskcontext.py	55.43%	41 Missing ⚠️
src/aiotools/taskscope.py	87.38%	14 Missing ⚠️
src/aiotools/utils.py	35.71%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #58      +/-   ##
==========================================
- Coverage   74.78%   74.00%   -0.79%     
==========================================
  Files          18       20       +2     
  Lines        1138     1250     +112     
==========================================
+ Hits          851      925      +74     
- Misses        287      325      +38     

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