Allow object as second i18n.getMessage() argument

[eemeli]

Currently, the second substitutions argument of i18n.getMessage() can be a single stringifiable value, or an array thereof. These substitutions may be referenced in a message or the placeholders as $1, $2, etc. To improve the translator experience, the placeholders object allows for these to be mapped to named placeholders in the message:

"notificationContent": {
  "message": "You clicked $URL$.",
  "description": "Tells the user which link they clicked.",
  "placeholders": {
    "url": {
      "content": "$1",
      "example": "https://developer.mozilla.org"
    }
  }
}

We could improve the legibility of the code formatting this message by allowing for substitutions to be an object, so that the above message could be formulated as:

"notificationContent": {
  "message": "You clicked $URL$.",
  "description": "Tells the user which link they clicked.",
  "examples": {
    "url": "https://developer.mozilla.org"
  }
}

and formatted with

browser.i18n.getMessage('notificationContent', { url: 'https://example.com' });

In case there's a concern about mis-identifying a substitution value that's intended to be stringified during formatting as instead an object from which substitution values are to be looked up, handling substitutions like this could be gated behind some flag as introduced in #698 for MF2 support. With that syntax, the above message could be formulated as:

"notificationContent": {
  "message": "You clicked {$url @example=|https://developer.mozilla.org|}.",
  "description": "Tells the user which link they clicked."
}