n-api: adds function to adjust external memory

Added a wrapper around
v8::Isolate::AdjustAmountOfExternalAllocatedMemory

PR-URL: https://github.com/nodejs/node/pull/14310


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Fixes: https://github.com/nodejs/node/issues/13928