Refactor wrapper

- Immediately create ipcMain so it doesn't have to be a function which I
  think feels cleaner.
  - Move exit handling to a separate function to compensate (otherwise
    the VS Code CLI for example won't be able to exit on its own).
- New isChild prop that is clearer than checking for parentPid (IMO).
- Skip all the checks that aren't necessary for the child process (like
  --help, --version, etc).
  - Since we check if we're the child in entry go ahead and move the
    wrap code into entry as well since that's basically what it does.
- Use a single catch at the end of the entry.
- Split out the VS Code CLI and existing instance code into separate
  functions.