src: make heap snapshot & embedder graph accessible for tests

Add methods that allow inspection of heap snapshots and a JS
version of our own embedder graph.

These can be used in tests and might also prove useful for
ad-hoc debugging. Usage requires `--expose-internals` and
prints a warning similar to our other modules whose primary
purpose is test support.

PR-URL: https://github.com/nodejs/node/pull/21741


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>