fix(js): tolerate missing File constructor

Guard URL-encoded form serialization so constrained browser contexts without File still submit generated hemx forms instead of throwing before fetch.

req: runtime/002

hemx-js/tests/runtime.rs

@@ -17,7 +17,7 @@ fn runtime_preserves_multipart_file_upload_fallback_shape() {
     assert!(source.contains(
         "multipart: form && String(form.enctype).toLowerCase() === \"multipart/form-data\""
     ));
-    assert!(source.contains("if (value instanceof File) continue"));
+    assert!(source.contains("if (typeof File !== \"undefined\" && value instanceof File) continue"));
     assert!(source.contains("return multipart ? data : urlEncoded(data)"));
     assert!(source.contains("if (body instanceof URLSearchParams) headers[\"Content-Type\"] = \"application/x-www-form-urlencoded;charset=UTF-8\""));
 }