style: use Unpack for better kwargs typing

[tlambert03]

this PR uses Unpack to provide nicer static IDE completion for all the places where widget kwargs are passed through a chain of super calls.

These changes have no runtime effects

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (2eaa144) 87.68% compared to head (e7e0cef) 87.85%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #599      +/-   ##
==========================================
+ Coverage   87.68%   87.85%   +0.16%     
==========================================
  Files          39       39              
  Lines        4573     4561      -12     
==========================================
- Hits         4010     4007       -3     
+ Misses        563      554       -9     

Files	Coverage Δ
src/magicgui/signature.py	96.47% <100.00%> (+0.04%)	⬆️
src/magicgui/type_map/_magicgui.py	96.15% <100.00%> (-0.08%)	⬇️
src/magicgui/widgets/_concrete.py	89.40% <100.00%> (+0.93%)	⬆️
src/magicgui/widgets/_table.py	96.14% <100.00%> (+<0.01%)	⬆️
src/magicgui/widgets/bases/_button_widget.py	100.00% <100.00%> (ø)
src/magicgui/widgets/bases/_categorical_widget.py	80.28% <ø> (ø)
src/magicgui/widgets/bases/_container_widget.py	91.13% <100.00%> (-0.05%)	⬇️
src/magicgui/widgets/bases/_ranged_widget.py	91.73% <100.00%> (+2.67%)	⬆️
src/magicgui/widgets/bases/_slider_widget.py	96.77% <100.00%> (ø)
src/magicgui/widgets/bases/_value_widget.py	98.33% <ø> (ø)
... and 1 more

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