[JSPI] - Assert that JSPI is supported by the environment. (emscripten-core#21483)

Author: brendandahl

src/library_async.js

@@ -40,6 +40,9 @@ addToLibrary({
     instrumentWasmImports(imports) {
 #if ASYNCIFY_DEBUG
       dbg('asyncify instrumenting imports');
+#endif
+#if ASSERTIONS && ASYNCIFY == 2
+      assert('Suspender' in WebAssembly, 'JSPI not supported by current environment. Perhaps it needs to be enabled via flags?');
 #endif
       var importPattern = {{{ new RegExp(`^(${ASYNCIFY_IMPORTS_EXCEPT_JS_LIBS.map(x => x.split('.')[1]).join('|').replace(/\*/g, '.*')})$`) }}};