message: fix(types): add Promise signature for bundleRenderer.renderToString
  * Make callback optional when providing context
  Otherwise TypeScript compiler complains about `not assignable to parameter of type 'RenderCallback'`
  * Update index.d.ts
diff: @@ -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;
  }
  