Add a feature flag to force dark mode

[zackkrida]

This issue is part of the Dark Mode project: #3592. Please see the implementation plan for additional detail and context.

Description

Create a new force_dark_mode feature flag, which allows developers to test dark mode without exposing the dark mode color palette to users.

The flag should have the following state:

{
   "force_dark_mode": {
      "status": {
        "staging": "switchable",
        "production": "disabled"
      },
      "defaultState": "off",
      "description": "Force the site to render in dark mode.",
      "storage": "session"
    }
}

This PR should also read the force_dark_mode feature flag and use its value to set a class on the body element (Please note that the original implementation plan reccommended the html element, but that the body is better-suited for visual classes like this). The original implementation plan didn't provide specifics into how to do this. For Nuxt 2 (and possibly Nuxt 3), we will likely want to use the head() method of each of our layout components to set the class like so:

head() {
  return {
    bodyAttrs: { class: featureFlagStore.isOn("force_dark_mode") ? "dark-mode" : "light-mode" }
  }
}

It may also, then, suit the scope of this PR to create a useDarkMode composable now to use in these layouts:

import { useFeatureFlagStore } from "~/stores/feature-flag"

/**
 * TODO: Replace with the user's actual dark mode preference.
 * Dark mode detection will be based on user preference,
 * overwritten by the "force_dark_mode" feature flag.
 */
export function useDarkMode() {
  const featureFlagStore = useFeatureFlagStore()
  const isDarkMode = featureFlagStore.isOn("force_dark_mode")
  return {
    isDarkMode,
    /** The CSS class representing the current color mode */
    cssClass: isDarkMode ? "dark-mode" : "light-mode",
  }
}

Implemented like so:

head() {
  const darkMode = useDarkMode()
  return {
    ...this.$nuxtI18nHead({
      addSeoAttributes: true,
      addDirAttribute: true,
    }),
    bodyAttrs: { class: darkMode.cssClass },
  }
}