path: deprecate internal _makeLong, replace

Replace the internal path._makeLong() with a public path.toNamespacedPath() method. Add documentation.

Refs: https://github.com/standard-things/esm/issues/66

Ping: @ljharb @jdalton

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

Affected core subsystem(s)

path