[3127] Where do custom directives belong?

I couldn't find anything in the guide about a best practice on this. It seems a custom directive could be a plain old javascript file with the Vue.directive(...) syntax but I don't see a clear winner for its directory or is the best practice to simply put the registration in the default.vue layout?

Just wanting to use this in my project:

// Register a global custom directive called `v-focus`
Vue.directive('focus', {
  // When the bound element is inserted into the DOM...
  inserted: function (el) {
    // Focus the element
    el.focus()
  }
})

Thanks!