Add .js extension to scripts

Since Node doesn't provide a way to cleanly run ESM programs¹, I choose
what seemed like the least-bad workaround and renamed the file to use a
.js extension.  This goes against best practice for program names, but
at least this is an internal-level thing where the impact is more
limited and we can symlink to avoid updating external references.

¹ nodejs/node#34049

Co-authored-by: Thomas Sibley <[email protected]>

Author: victorlin