feat: raise_synchronous option, and expose app classes

[tlambert03]

makes it easier for applications to override their registry classes, and also adds a raise_synchronous_exceptions option (defaults to False) that, when True, will raise any exceptions encountered during execute_command in a synchronous context

[codecov]

Codecov Report

Merging #40 (1a67067) into main (b3f1a10) will decrease coverage by 0.10%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##             main      #40      +/-   ##
==========================================
- Coverage   97.53%   97.42%   -0.11%     
==========================================
  Files          31       31              
  Lines        1621     1630       +9     
==========================================
+ Hits         1581     1588       +7     
- Misses         40       42       +2     

Impacted Files	Coverage Δ
src/app_model/_app.py	94.91% <83.33%> (-3.20%)	⬇️
src/app_model/registries/_commands_reg.py	94.44% <100.00%> (+0.24%)	⬆️

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