Add support for non-returning handlers

[viceroypenguin]

Fixes #186

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
...Immediate.Apis.Generators/ITypeSymbolExtensions.cs	95.77% <100.00%> (+1.37%)	⬆️
...e.Apis.Generators/ImmediateApisGenerator.Models.cs	100.00% <100.00%> (ø)
...pis.Generators/ImmediateApisGenerator.Transform.cs	90.62% <100.00%> (+0.75%)	⬆️

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