refactor: make ValueWidget a Generic

[tlambert03]

adds Generic typing to ValueWidget

[codecov]

Codecov Report

Base: 89.77% // Head: 89.78% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (553f2a4) compared to base (05dd1a1).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #517      +/-   ##
==========================================
+ Coverage   89.77%   89.78%   +0.01%     
==========================================
  Files          37       37              
  Lines        4450     4456       +6     
==========================================
+ Hits         3995     4001       +6     
  Misses        455      455              

Impacted Files	Coverage Δ
src/magicgui/type_map/_type_map.py	96.48% <ø> (ø)
src/magicgui/widgets/_concrete.py	89.10% <100.00%> (+0.04%)	⬆️
src/magicgui/widgets/_function_gui.py	93.75% <100.00%> (ø)
src/magicgui/widgets/bases/_button_widget.py	100.00% <100.00%> (ø)
src/magicgui/widgets/bases/_categorical_widget.py	84.00% <100.00%> (ø)
src/magicgui/widgets/bases/_create_widget.py	97.50% <100.00%> (ø)
src/magicgui/widgets/bases/_ranged_widget.py	88.23% <100.00%> (ø)
src/magicgui/widgets/bases/_slider_widget.py	96.96% <100.00%> (+0.30%)	⬆️
src/magicgui/widgets/bases/_value_widget.py	98.36% <100.00%> (+0.02%)	⬆️

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