src: port coverage serialization to C++

This patch moves the serialization of coverage profiles into
C++. With this we no longer need to patch `process.reallyExit`
and hook into the exit events, but instead hook into relevant
places in C++ which are safe from user manipulation. This also
makes the code easier to reuse for other types of profiles.

PR-URL: https://github.com/nodejs/node/pull/26874


Reviewed-By: Ben Coe <bencoe@gmail.com>