Insert async keyword as last modifier (#27491)

src/testRunner/unittests/convertToAsyncFunction.ts

@@ -1254,6 +1254,11 @@ function [#|main2|]() {
         .then(() => { console.log("."); return delay(500); })
         .then(() => { console.log("."); return delay(500); })
 }
+`);
+_testConvertToAsyncFunction("convertToAsyncFunction_exportModifier", `
+export function [#|foo|]() {
+    return fetch('https://typescriptlang.org').then(s => console.log(s));
+}
 `);
     });