[Feature Request] support sending "Building..." when HMR trigger

[rezio-gary]

Here is my task.json: When I started this task, vscode would be able to tell me is storybook is re-building the pack. I hope this behave can be implemented in the internal storybook server.

reference

{
  "version": "2.0.0",
  "tasks": [
    {
      "isBackground": true,
      "type": "npm",
      "script": "storybook",
      "problemMatcher": [
        {
          "owner": "typescript",
          "fileLocation": "relative",
          "pattern": {
            "regexp": ".*\\s+(error|warning|info)\\s*:\\s*(.*)$",
            "location": 2,
            "severity": 3,
            "code": 4,
            "message": 5
          },
          "background": {
            "activeOnStart": true,
            "beginsPattern": ".*webpack building...",
            "endsPattern": ".*99% done plugins().*"
          }
        }
      ],
      "label": "storybook",
      "detail": "start-storybook -p 6006",
      "group": {
        "kind": "build",
        "isDefault": true
      }
    }
  ]
}