Sync custom editor edits

If multiple instances of the same  custom editor are opened for the same resource, the edit stack should be shared between them. This matches how we work with text files

src/vs/workbench/api/browser/mainThreadWebview.ts

@@ -289,7 +289,7 @@ export class MainThreadWebviews extends Disposable implements extHostProtocol.Ma
 
 				try {
 					await this._proxy.$resolveWebviewEditor(
-						webviewInput.getResource(),
+						{ resource: webviewInput.getResource(), edits: model.currentEdits },
 						handle,
 						viewType,
 						webviewInput.getTitle(),
@@ -299,6 +299,7 @@ export class MainThreadWebviews extends Disposable implements extHostProtocol.Ma
 				} catch (error) {
 					onUnexpectedError(error);
 					webviewInput.webview.html = MainThreadWebviews.getDeserializationFailedContents(viewType);
+					return;
 				}
 			}
 		}));