add tests, improve error return from assertUnreachable

[Kalininator]

No description provided.

[codecov]

Codecov Report

Merging #20 (e67a92c) into master (e839e75) will increase coverage by 3.14%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
+ Coverage   86.85%   90.00%   +3.14%     
==========================================
  Files          16       16              
  Lines         289      290       +1     
  Branches       50       50              
==========================================
+ Hits          251      261      +10     
+ Misses         38       29       -9     

Impacted Files	Coverage Δ
src/utils.ts	100.00% <100.00%> (+20.00%)	⬆️
src/chrome/ChromeWindowsCookieProvider.ts	100.00% <0.00%> (+15.38%)	⬆️
src/index.ts	94.28% <0.00%> (+17.14%)	⬆️

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