inspector: report when main context is destroyed

PR-URL: https://github.com/nodejs/node/pull/12814
Reimplements: https://github.com/nodejs/node/pull/7756
Fixes: https://github.com/nodejs/node/issues/7742


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>