fix: node 6 compatibility

[ricokahler]

Closes #351
Closes #352

[codecov]

Codecov Report

Merging #353 (baf3fd5) into main (aecf433) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #353      +/-   ##
==========================================
+ Coverage   96.29%   96.39%   +0.09%     
==========================================
  Files          23       23              
  Lines         189      194       +5     
  Branches       41       41              
==========================================
+ Hits          182      187       +5     
  Misses          7        7              

Impacted Files	Coverage Δ
src/getLuminance.ts	100.00% <100.00%> (ø)
src/parseToRgba.ts	94.73% <100.00%> (+0.29%)	⬆️

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[github-actions]

🎉 This PR is included in version 1.2.3 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀

[ljharb]

This fails to account for https://github.com/ricokahler/color2k/blob/5e3bde38bf674b595343983c46d71ccf6867563e/src/toHex.ts