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/browser/mainThreadWebview.ts

@@ -150,7 +150,7 @@ export class MainThreadWebviews extends Disposable implements MainThreadWebviews
 		webview.setName(value);
 	}
 
-	public $setState(handle: WebviewPanelHandle, state: modes.WebviewEditorState): void {
+	public $setState(handle: WebviewPanelHandle, state: modes.WebviewContentState): void {
 		const webview = this.getWebviewEditorInput(handle);
 		if (webview instanceof CustomFileEditorInput) {
 			webview.setState(state);