feat: allow extending signals

[nank1ro]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (d05918f) to head (2e08a54).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #130   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           17        17           
  Lines          937       921   -16     
=========================================
- Hits           937       921   -16     

Files with missing lines	Coverage Δ
...ckages/solidart/lib/src/core/collections/list.dart	100.00% <100.00%> (ø)
...ackages/solidart/lib/src/core/collections/map.dart	100.00% <100.00%> (ø)
...ackages/solidart/lib/src/core/collections/set.dart	100.00% <100.00%> (ø)
packages/solidart/lib/src/core/computed.dart	100.00% <100.00%> (ø)
packages/solidart/lib/src/core/read_signal.dart	100.00% <100.00%> (ø)
packages/solidart/lib/src/core/resource.dart	100.00% <100.00%> (ø)
packages/solidart/lib/src/core/signal.dart	100.00% <100.00%> (ø)

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