fixed warnings from eslint

src/types/fetch.d.ts

@@ -0,0 +1,6 @@
+// Node's fetch needs `duplex: 'half'` for stream bodies,
+// but lib.dom's RequestInit doesn't declare it.
+// This interface is automatically merged with RequestInit from lib.dom.d.ts
+interface RequestInit {
+    duplex?: 'half'
+}