vscode api: Raise compatibility to webview content

The webview (or its  `acquireVsCodeApi` injector)
adds the vscode api but also deletes the properties
`window.parent`, `window.top` and `window.frameElement`.

This voilates the HTML Spec and makes problems with
some code like `if (window.parent === window)` to
detect if we are running in an iframe.

Yes, this affects Hyrum's Law and https://xkcd.com/1172/
but detecting `acquireVsCodeApi` object itself feels to be cleaner anyway.

Author: HolgerJeromin

src/vs/workbench/contrib/webview/browser/pre/index.html

@@ -221,9 +221,9 @@
 							});
 						};
 					})();
-					delete window.parent;
-					delete window.top;
-					delete window.frameElement;
+					window.parent = window;
+					window.top = window;
+					window.frameElement = null;
 				`;
 		}