Add support for setting default nullability for fields and return types

[MichalLytek]

Closes #297 🎉

[codecov]

Codecov Report

Merging #302 into master will increase coverage by 0.12%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #302      +/-   ##
==========================================
+ Coverage    94.2%   94.32%   +0.12%     
==========================================
  Files          69       69              
  Lines         967      970       +3     
  Branches       73       72       -1     
==========================================
+ Hits          911      915       +4     
+ Misses         53       52       -1     
  Partials        3        3

Impacted Files	Coverage Δ
src/helpers/types.ts	97.5% <100%> (+2.37%)	⬆️
src/schema/build-context.ts	100% <100%> (ø)	⬆️
src/schema/schema-generator.ts	96.27% <100%> (+0.04%)	⬆️

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