Renames the --loader cli argument to --experimental-loader. This is
to clearly indicate the esm loader feature as experimental even after
esm is no longer experimental.

Also minorly alters the --experimental-loader docs to say that the
passed loader can be an esm module.

An alternative to this PR is to leave things unchanged and continue to use --experimental-modules to allow the use of --loader, even after ECMAScript modules are unflagged. This would reduce the number of different argument schemes a hypothetical script that wraps node would need to try, as discussed here: nodejs/modules#351 (comment)

Refs: nodejs/modules#351 (comment)

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