Add experimental.useBackgroundImageForWindow to schema (#13114)

Adds the `experimental.useBackgroundImageForWindow` global setting introduced in #12893 to the schema.

(cherry picked from commit b699f92)
Service-Card-Id: 82002294
Service-Version: 1.14

doc/cascadia/profiles.schema.json

@@ -1708,6 +1708,11 @@
           "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.",