src: turn inspector raw pointer into unique_ptr

Use a unique pointer to make ownership clear.

PR-URL: https://github.com/nodejs/node/pull/16974


Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Eugene Ostroukhov <eostroukhov@google.com>