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/Request.idl

@@ -6,7 +6,7 @@
 typedef (Request or USVString) RequestInfo;
 
 // https://fetch.spec.whatwg.org/#request
-[Exposed=(Window,Worker)]
+[Exposed=(Window,Worker), UseNewAKString]
 interface Request {
     constructor(RequestInfo input, optional RequestInit init = {});