LibWeb: Implement Headers.getSetCookie()

This is a normative change in the Fetch spec.
See: e4d3480

This also implements the changes to the 'sort and combine' algorithm,
which now treats "set-cookie" headers differently, and is exposed to JS
via the Headers' iterator.

Passes all 21 WPT tests :^)
http://wpt.live/fetch/api/headers/header-setcookie.any.html

Userland/Libraries/LibWeb/Fetch/Headers.idl

@@ -7,6 +7,7 @@ interface Headers {
     undefined append(ByteString name, ByteString value);
     undefined delete(ByteString name);
     ByteString? get(ByteString name);
+    sequence<ByteString> getSetCookie();
     boolean has(ByteString name);
     undefined set(ByteString name, ByteString value);
     iterable<ByteString, ByteString>;