vm: allow modifying context name in inspector

The `auxData` field is not exposed to JavaScript, as DevTools uses it
for its `isDefault` parameter, which is implemented faithfully,
contributing to the nice indentation in the context selection panel.
Without the indentation, when `Target` domain gets implemented (along
with a single Inspector for cluster) in #16627, subprocesses and VM
contexts will be mixed up, causing confusion.

PR-URL: https://github.com/nodejs/node/pull/17720
Refs: https://github.com/nodejs/node/pull/14231#issuecomment-315924067


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: James M Snell <jasnell@gmail.com>