[wip] src, inspector: support opted-in VM contexts

Works, doesn't crash, but no tests or docs at this moment.

V8's console is injected unconditionally upon every attachContext call, which may be not desirable.

Uses an approach inspired by @jgoz in https://github.com/nodejs/node/issues/7593#issuecomment-310128400.

/cc @eugeneo @bmeck

Fixes: nodejs#7593 Refs: nodejs#9272

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)

inspector