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/Response.idl

@@ -2,7 +2,7 @@
 #import <Fetch/BodyInit.idl>
 #import <Fetch/Headers.idl>
 
-[Exposed=(Window,Worker)]
+[Exposed=(Window,Worker), UseNewAKString]
 interface Response {
     constructor(optional BodyInit? body = null, optional ResponseInit init = {});