Detect and warn for invalid configuration of the `release-it` builtin `npm` plugin

[rwjblue-glean]

See if we can provide better feedback if we detect that the end user hasn't completely disabled the npm plugin. Right now, if you do something like:

{
  "release-it": {
    "npm": {
      "publish": false
    }
  }
}

You end up with a really bad time. Poor error message, without much recourse, &c.