src: move per-process global variables into node::per_process

So that it's easier to tell whether we are manipulating per-process
global states that may need to be treated with care to avoid races.

Also added comments about these variables and moved some of them
to a more suitable compilation unit:

- Move `v8_initialized` to `util.h` since it's only used in
  `util.cc` and `node.cc`
- Rename `process_mutex` to `tty_mutex` and move it into
  `node_errors.cc` since that's the only place it's used
  to guard the tty.
- Move `per_process_opts_mutex` and `per_process_opts`
  into `node_options.h` and rename them to
  `per_process::cli_options[_mutex]`
- Rename `node_isolate[_mutex]` to `per_process::main_isolate[_mutex]`

PR-URL: https://github.com/nodejs/node/pull/25302


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>