message: Add `experimental.useBackgroundImageForWindow` to schema
  Adds the `experimental.useBackgroundImageForWindow` global setting introduced in to the schema.
diff: "description": "Force the terminal to use the legacy input encoding. Certain keys in some applications may stop working when enabling this setting.",
  "type": "boolean"
  },
  + "experimental.useBackgroundImageForWindow": {
  + "default": false,
  + "description": "When set to true, the background image for the currently focused profile is expanded to encompass the entire window, beneath other panes.",
  + "type": "boolean"
  + },
  "initialCols": {
  "default": 120,
  "description": "The number of columns displayed in the window upon first load. If \"launchMode\" is set to \"maximized\" (or \"maximizedFocus\"), this property is ignored.",
  