Provide custom instruction in Vision for Copilot

[doggy8088]

Is it possible to configure custom instructions for this extension? It will be helpful in many situations.

For example:

{
  "copilot.vision.instructions": [
    {
        "file": ".copilot-vision-instructions.md"
    }
  ]
}

or

{
  "copilot.vision.instructions": [
    {
        "text": "Write text only in #zh-tw"
    }
  ]
}