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 (closed), subprocesses and VM contexts will be mixed up, causing confusion.

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

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)

vm, inspector, src