feat: add quantity widget

[tlambert03]

Adds a widget (qt backend only for now) for pint.Quantity types

[codecov]

Codecov Report

Merging #483 (9ca7ac4) into main (340fce9) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #483   +/-   ##
=======================================
  Coverage   89.53%   89.54%           
=======================================
  Files          31       31           
  Lines        3977     3987   +10     
=======================================
+ Hits         3561     3570    +9     
- Misses        416      417    +1     

Impacted Files	Coverage Δ
magicgui/backends/_qtpy/__init__.py	100.00% <ø> (ø)
magicgui/widgets/__init__.py	100.00% <ø> (ø)
magicgui/backends/_qtpy/widgets.py	87.88% <100.00%> (+0.05%)	⬆️
magicgui/type_map.py	96.61% <100.00%> (+0.06%)	⬆️
magicgui/widgets/_concrete.py	87.45% <100.00%> (+0.04%)	⬆️
magicgui/_type_resolution.py	91.52% <0.00%> (-1.70%)	⬇️

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