Allow attaching variable ids to chat open command args

[Tyriar]

Part of microsoft/vscode-copilot#10125

Tested with:

    {
        "command": "workbench.action.chat.open",
        "args": {
            "variableIds": [
                "copilot.terminalSelection"
            ],
            "isPartialQuery": true
        },
        "key": "ctrl+shift+c"
    }