src: simplify Environment::HandleCleanup

- Make the HandleCleanup a struct, and make the queue a
  std::list, iterate over it in CleanupHandles() and clear
  it after that.
- Put the handle cleanup registration into a method and
  document that they will not be called in the one environemt
  per process setup.

PR-URL: https://github.com/nodejs/node/pull/19319


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Jackson Tian <shyvo1987@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>