process: refactor coverage setup during bootstrap

- Renamed `internal/process/write-coverage.js` to
  `internal/coverage-gen/with_instrumentation.js`,
  `internal/process/coverage.js` to
  `internal/coverage-gen/with_profiler.js` to distinguish
  the two better and added comments.
- Separate the coverage directory setup and the connection
  setup, moves the directory setup into `node.js` and
  closer to the exit hooks because that's where it's used.
- Moves the `process.reallyExit` overwrite and
  `process.on('exit')` hooks setup into bootstrap/node.js
  for clarity, and move them to a later stage of
  bootstrap since they do not have to happen that early.

PR-URL: https://github.com/nodejs/node/pull/25398


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