add typing support for fields

[NiklasKappel]

Node constructors already work with the addition of 'dataclass_transform' in ZnInit.

For the field specifier functions we could lie about the return type. If they don't take arguments (e.g. outs) that's easy, because we just annotate a return type of Any. Otherwise, they seem to forward all arguments to the __init__ of ZnInit's Descriptor, so we could use the exact same type hints as for the desc factory function.

This way we would need boilerplate type stubs for all functions in fields.py. I don't know if it is possible to add type hints for the constructor (__init__ or __new__??) of Descriptor directly and propagate that through the inheritance chain.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.82%. Comparing base (6698c4a) to head (637ae0d).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #767      +/-   ##
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- Coverage   91.01%   86.82%   -4.19%     
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  Files          25       25              
  Lines        2047     2057      +10     
  Branches      974      979       +5     
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- Hits         1863     1786      -77     
- Misses        184      271      +87     

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fulltest	86.82% <ø> (+0.06%)	⬆️

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