LibWeb/Fetch: Port JS interfaces to new String

Since BodyInit and Headers are tightly coupled to both Request and
Response, I chose to do all of them at once instead of introducing a
bunch of temporary conversion glue code.

Userland/Libraries/LibWeb/Fetch/Headers.idl

@@ -1,6 +1,6 @@
 typedef (sequence<sequence<ByteString>> or record<ByteString, ByteString>) HeadersInit;
 
-[Exposed=(Window,Worker)]
+[Exposed=(Window,Worker), UseNewAKString]
 interface Headers {
     constructor(optional HeadersInit init);