esm: treat modules with no extension as .js

with this change, extensionless modules loaded with esm loader will be loaded with the same behavior as .js files:

1. if "type": "module" is specified, load as an esm module
2. otherwise, load as cjs module

Fixes: https://github.com/nodejs/node/issues/34049 Fixes: https://github.com/nodejs/node/issues/33226

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