Set minimum Node version to 12

[kachkaev]

Node 10 reached end of life in April 2021. See https://nodejs.org/en/about/releases/

[codecov]

Codecov Report

Merging #25 (17af91a) into main (d688cec) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #25   +/-   ##
=======================================
  Coverage   92.63%   92.63%           
=======================================
  Files           5        5           
  Lines          95       95           
  Branches        9        9           
=======================================
  Hits           88       88           
  Misses          5        5           
  Partials        2        2           

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