feat: Combo Option to add item at the top of the list

[abasse]

Added a new option "addtotop" to the combo component to be able to add new items at the top.
Default is false.

{
  "type": "page",
  "body": {
    "type": "form",
    "debug": true,
    "mode": "horizontal",
    "api": "/api/mock2/form/saveForm",
    "body": [
      {
        "type": "combo",
        "name": "combo1",
        "label": "默认模式",
        "multiple": true,
        "addtotop": true,
        "items": [
          {
            "name": "text",
            "type": "input-text"
          }
        ]
      },
      {
        "type": "divider"
      },
      {
        "type": "combo",
        "name": "combo2",
        "label": "打平模式",
        "multiple": true,
        "addtotop": true,
        "flat": true,
        "items": [
          {
            "name": "text",
            "type": "input-text"
          }
        ]
      }
    ]
  }
}

[github-actions]

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