fix: accept thenable bootstrapFunction return values (#574) (#575)

Author: kfrederix

libs/single-spa-angular/src/single-spa-angular.ts

@@ -1,4 +1,4 @@
-import type { ApplicationRef, NgModuleRef, NgZone } from '@angular/core';
+import { ɵisPromise, type ApplicationRef, type NgModuleRef, type NgZone } from '@angular/core';
 import type { LifeCycles } from 'single-spa';
 import { getContainerElementAndSetTemplate } from 'single-spa-angular/internals';
 
@@ -99,7 +99,7 @@ async function mount(
 
   const bootstrapPromise = options.bootstrapFunction(props);
 
-  if (!(bootstrapPromise instanceof Promise)) {
+  if (!ɵisPromise(bootstrapPromise)) {
     throw Error(
       `single-spa-angular: the options.bootstrapFunction must return a promise, but instead returned a '${typeof bootstrapPromise}' that is not a Promise`,
     );