fix(types): add missing ssr renderToString signature

packages/vue-server-renderer/types/index.d.ts

@@ -11,6 +11,7 @@ interface Renderer {
   renderToString(vm: Vue, callback: RenderCallback): void;
   renderToString(vm: Vue, context: object, callback: RenderCallback): void;
   renderToString(vm: Vue): Promise<string>;
+  renderToString(vm: Vue, context: object): Promise<string>;
 
   renderToStream(vm: Vue, context?: object): Readable;
 }

types/test/ssr-test.ts

@@ -46,6 +46,14 @@ renderer.renderToString(app)
     throw err;
   });
 
+renderer.renderToString(app, context)
+  .then(html => {
+    const res: string = html;
+  })
+  .catch(err => {
+    throw err;
+  });
+
 renderer.renderToStream(app, context).on('data', chunk => {
   const html = chunk.toString();
 });