feat: add injection model to app

[tlambert03]

No description provided.

[codecov]

Codecov Report

Merging #21 (c4d6af4) into main (51c2f90) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #21   +/-   ##
=======================================
  Coverage   99.63%   99.63%           
=======================================
  Files          28       28           
  Lines        1374     1381    +7     
=======================================
+ Hits         1369     1376    +7     
  Misses          5        5           

Impacted Files	Coverage Δ
src/app_model/backends/qt/_qmenu.py	100.00% <ø> (ø)
src/app_model/_app.py	100.00% <100.00%> (ø)
src/app_model/registries/_commands_reg.py	97.01% <100.00%> (+0.13%)	⬆️
src/app_model/registries/_menus_reg.py	100.00% <100.00%> (ø)
src/app_model/registries/_register.py	100.00% <100.00%> (ø)

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