Use overload for #62806

Use an overload instead of a conditional type to generate a compile error when using `vscode.previewHtml` command

Conditional types were breaking on our build machine

src/vs/vscode.d.ts

@@ -5840,6 +5840,7 @@ declare module 'vscode' {
 		 * the command handler function doesn't return anything.
 		 */
 		export function executeCommand<T>(command: string, ...rest: any[]): Thenable<T | undefined>;
+		export function executeCommand<T>(command: 'vscode.previewHtml', error: { '⚠️ The vscode.previewHtml command is deprecated and will be removed. Please switch to using the Webview Api': never }, ...rest: any[]): Thenable<T | undefined>;
 
 		/**
 		 * Retrieve the list of all available commands. Commands starting an underscore are