[product icon themes] theme icons support for vscode.show(Information|Warning|Error)Message

<!-- ⚠️⚠️ Do Not Delete This! feature_request_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- Please search existing issues to avoid creating duplicates. --> <!-- Describe the feature you'd like. --> The methods `vscode.show(Information|Warning|Error)Message` do not support theme icons `$(<name>)`-syntax, although the notification box shows a theme icon in front of the message.