Move webview content state into webviewPanel instead of webviewEditor

Split out from #77131

The current webview editor api is very procedural. This model has some problems when it comes to supporting editing resources, but actually does make a lot of sense for webviews that aren't backed by real file system resources. For example, if you have a webview that edits some document in the cloud, you should not be required to implement a custom file system provider just to enable basic saving.

This change moves the `onWillSave` and `webviewEditorState` properties back onto `WebviewPanel` instead of keeping them specific to `WebviewEditor`. The save implementation does not fully work yet, as the will require #81521

src/vs/workbench/api/common/extHost.api.impl.ts

@@ -909,7 +909,7 @@ export function createApiFactoryAndRegisterActors(accessor: ServicesAccessor): I
 			CallHierarchyItem: extHostTypes.CallHierarchyItem,
 			DebugConsoleMode: extHostTypes.DebugConsoleMode,
 			Decoration: extHostTypes.Decoration,
-			WebviewEditorState: extHostTypes.WebviewEditorState,
+			WebviewContentState: extHostTypes.WebviewContentState,
 			UIKind: UIKind
 		};
 	};