src/vs/workbench/api/common/extHostTypes.ts/MarkdownString/isMarkdownString
class MarkdownString:	static isMarkdownString(thing: any): thing is vscode.MarkdownString {
		if (thing instanceof MarkdownString) {
			return true;
		}
		return thing && thing.appendCodeblock && thing.appendMarkdown && thing.appendText && (thing.value !== undefined);
	}