Improve AsyncState

[mcmah309]

closes #351

merge #354 first

[codecov]

Codecov Report

Attention: Patch coverage is 78.57143% with 21 lines in your changes missing coverage. Please review.

Project coverage is 83.84%. Comparing base (dd5c6ec) to head (c6cff56).
Report is 126 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/signals_core/lib/src/async/state.dart	77.41%	21 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #355       +/-   ##
===========================================
- Coverage   98.82%   83.84%   -14.99%     
===========================================
  Files          38       60       +22     
  Lines        1196     2557     +1361     
===========================================
+ Hits         1182     2144      +962     
- Misses         14      413      +399     

Files with missing lines	Coverage Δ
packages/signals_core/lib/src/async/signal.dart	100.00% <100.00%> (ø)
packages/signals_core/lib/src/core/computed.dart	97.39% <100.00%> (-0.39%)	⬇️
packages/signals_core/lib/src/core/signal.dart	98.50% <ø> (+1.53%)	⬆️
packages/signals_core/lib/src/async/state.dart	86.06% <77.41%> (-13.94%)	⬇️

... and 31 files with indirect coverage changes

[rodydavis]

LGTM!