stream_wrap: export `_externalUVStream`

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)

stream_wrap

Description of change

This can be hooked up with user-land libuv modules, and possibly with uv_link_t later on.