build: Set target to ES2019

[peaceiris]

cf. Node Target Mapping · microsoft/TypeScript Wiki

[codecov]

Codecov Report

Merging #576 (ba92668) into main (94fb28f) will decrease coverage by 0.36%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #576      +/-   ##
==========================================
- Coverage   61.17%   60.80%   -0.37%     
==========================================
  Files           4        4              
  Lines         273      273              
  Branches       48       48              
==========================================
- Hits          167      166       -1     
- Misses        105      106       +1     
  Partials        1        1              

Impacted Files	Coverage Δ
src/utils.ts	97.61% <0.00%> (-2.39%)	⬇️

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