feat: eval expr when creating menus

[tlambert03]

this makes it possible to declare an action or menu item with enablement: False (perhaps based on an environment variable or setting or something) to immediately hide it

[codecov]

Codecov Report

Merging #61 (59e8e83) into main (a50fe82) will decrease coverage by 0.11%.
The diff coverage is 81.81%.

@@            Coverage Diff             @@
##             main      #61      +/-   ##
==========================================
- Coverage   99.46%   99.34%   -0.12%     
==========================================
  Files          31       31              
  Lines        1682     1688       +6     
==========================================
+ Hits         1673     1677       +4     
- Misses          9       11       +2     

Impacted Files	Coverage Δ
src/app_model/expressions/_expressions.py	99.02% <75.00%> (-0.98%)	⬇️
src/app_model/backends/qt/_qaction.py	100.00% <100.00%> (ø)
src/app_model/types/_command_rule.py	100.00% <100.00%> (ø)

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