Clean up markdown preview messaging (#176458)

- Add properties directly to message
- Add `ImageInfo` type
- Don't use state to pass around imageInfo

extensions/markdown-language-features/preview-src/messaging.ts

@@ -25,7 +25,7 @@ export const createPosterForVsCode = (vscode: any, settingsManager: SettingsMana
 			vscode.postMessage({
 				type,
 				source: settingsManager.settings!.source,
-				body
+				...body
 			});
 		}
 	};