fix: return 'false' and 'null' values as JSON strings

[danielroe]

resolves #32

[codecov-commenter]

Codecov Report

Merging #33 (7a364b5) into main (2576b2a) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #33   +/-   ##
=======================================
  Coverage   94.05%   94.05%           
=======================================
  Files          10       10           
  Lines         185      185           
  Branches       50       50           
=======================================
  Hits          174      174           
  Misses         11       11           

Impacted Files	Coverage Δ
src/app.ts	96.66% <100.00%> (ø)

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