fix: treat compilation.errors as a set

[guybedford]

I believe this will fix the deprecation warning in #704, by not assuming compilation.errors has array methods. Resolves #704.

[codecov-commenter]

Codecov Report

Merging #705 (3ff903e) into main (571eced) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main     #705   +/-   ##
=======================================
  Coverage   73.52%   73.52%           
=======================================
  Files          13       13           
  Lines         476      476           
=======================================
  Hits          350      350           
  Misses        126      126           

Impacted Files	Coverage Δ
src/index.js	79.49% <0.00%> (ø)

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