fix(types): add Promise signature for bundleRenderer.renderToString (#7098)

* Make callback optional when providing context

Otherwise TypeScript compiler complains about `not assignable to parameter of type 'RenderCallback'`

* Update index.d.ts

Author: sleewoo

Diff for: packages/vue-server-renderer/types/index.d.ts

@@ -18,6 +18,7 @@ interface Renderer {
 interface BundleRenderer {
   renderToString(callback: RenderCallback): void;
   renderToString(context: object, callback: RenderCallback): void;
+  renderToString(context: object): Promise<string>;
 
   renderToStream(context?: object): Readable;
 }