fix(builder): restore resolution of .mjs files

[merceyz]

What's the problem this PR addresses?

#4732 stopped resolving .mjs files by default which probably wasn't on purpose.

How did you fix it?

Added resolveExtensions with the default extensions and .mjs.

Results

The performance doesn't change but the bundle size is slightly improved

$ du -s yarn*
2736    yarn-after.js
2744    yarn-before.js

$ YARN_IGNORE_PATH=1 hyperfine -w 1 \
  --prepare 'node ./before.cjs remove dummy-pkg || true' \
  'node ./before.cjs add dummy-pkg@link:./dummy-pkg' \
  --prepare 'node ./after.cjs remove dummy-pkg || true' \
  'node ./after.cjs add dummy-pkg@link:./dummy-pkg'
Benchmark 1: node ./before.cjs add dummy-pkg@link:./dummy-pkg
  Time (mean ± σ):      1.280 s ±  0.017 s    [User: 1.724 s, System: 0.306 s]
  Range (min … max):    1.263 s …  1.316 s    10 runs

Benchmark 2: node ./after.cjs add dummy-pkg@link:./dummy-pkg
  Time (mean ± σ):      1.278 s ±  0.007 s    [User: 1.699 s, System: 0.320 s]
  Range (min … max):    1.271 s …  1.289 s    10 runs

Summary
  'node ./after.cjs add dummy-pkg@link:./dummy-pkg' ran
    1.00 ± 0.01 times faster than 'node ./before.cjs add dummy-pkg@link:./dummy-pkg'

Checklist

• I have read the Contributing Guide.
• I have set the packages that need to be released for my changes to be effective.
• I will check that all automated PR checks pass before the PR gets reviewed.