Add type annotations for properties defined by get/set

[mzeiher]

This is a quick proof of concept on how to add type annotations for get/set properties, I could also add tests and cleanup as soon as I know how to your package is exactly organized :)

[codecov]

Codecov Report

Merging #6 into master will decrease coverage by 5%.
The diff coverage is 58.82%.

@@            Coverage Diff             @@
##           master       #6      +/-   ##
==========================================
- Coverage   91.89%   86.88%   -5.01%     
==========================================
  Files           4        4              
  Lines         111      122      +11     
  Branches       41       46       +5     
==========================================
+ Hits          102      106       +4     
- Misses          9       16       +7

Impacted Files	Coverage Δ
src/metadata/metadataVisitor.ts	78.94% <57.14%> (-13.91%)	⬇️
src/metadata/serializeType.ts	85.13% <60%> (-4.58%)	⬇️

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