test: run on multiple node versions

[wardpeet]

Added node matrix to github actions. This will make sure we keep compat if features like flatMap will be used.

[codecov[bot]]

Codecov Report

Merging #306 into master will decrease coverage by 0.18%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #306      +/-   ##
==========================================
- Coverage   92.68%   92.50%   -0.19%     
==========================================
  Files          10       10              
  Lines         328      320       -8     
  Branches      115      113       -2     
==========================================
- Hits          304      296       -8     
  Misses         23       23              
  Partials        1        1              

Impacted Files	Coverage Δ
src/index.js	98.97% <0.00%> (-0.08%)	:arrow_down:

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