message: schema: add support for "auto" split value
diff: "SplitState": {
  "enum": [
  "vertical",
  - "horizontal"
  + "horizontal",
  + "auto"
  ],
  "type": "string"
  },
  "action": { "type": "string", "pattern": "splitPane" },
  "split": {
  "$ref": "#/definitions/SplitState",
  - "default": "vertical",
  - "description": "The orientation to split the pane in, either vertical (think [|]) or horizontal (think [-])"
  + "default": "auto",
  + "description": "The orientation to split the pane in, either vertical (think [|]), horizontal (think [-]), or auto (splits pane based on remaining space)"
  }
  }
  }
  