chore!: drop support for Node.js v12

[wellwelwel]

Checking performance.

---

✅ The increase was really significant, running almost 1x (~5.99x) faster than Jest, for example.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.51%. Comparing base (46c735a) to head (5f7e2f4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #896   +/-   ##
=======================================
  Coverage   98.51%   98.51%           
=======================================
  Files          35       35           
  Lines        1752     1752           
  Branches       17       17           
=======================================
  Hits         1726     1726           
  Misses         11       11           
  Partials       15       15           

Flag	Coverage Δ
linux	98.40% <100.00%> (ø)
osx	98.45% <100.00%> (ø)
windows	98.40% <100.00%> (ø)

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