Switch webview api back to use delegate model

For #77131

Going back the the delegate based model for a few reasons:

- It gives us a better approach to add additional API hooks in the future (such as for rename)

- In practive, the capabilities were almost always the same as the `userData` on the document. It is rather confusing to have both `userData` and the capabilities 'on' the document

extensions/markdown-language-features/src/features/previewManager.ts

@@ -148,8 +148,8 @@ export class MarkdownPreviewManager extends Disposable implements vscode.Webview
 		this.registerDynamicPreview(preview);
 	}
 
-	public async resolveCustomDocument(_document: vscode.CustomDocument): Promise<vscode.CustomEditorCapabilities> {
-		return {};
+	public async resolveCustomDocument(_document: vscode.CustomDocument): Promise<void> {
+		// noop
 	}
 
 	public async resolveCustomTextEditor(