build: add V8 embedder version string

After this commit, `process.versions.v8` will look like:
"6.0.287.53-node.0".
The goal is that everytime we apply a non-official patch to `deps/v8`,
we increment our own number instead of V8's patch level.
This number must be set back to 0 after major V8 updates.

Fixes: https://github.com/nodejs/node/issues/15698
PR-URL: https://github.com/nodejs/node/pull/15785


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>