doc: use named import and Set for builtinModules

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

just using named import for good measure, as well as using a Set, to be consistent with:

const JS_EXTENSIONS = new Set(['.js', '.mjs']);