LibWeb: Add referrer policy to Fetch::Infrastructure::Request

The enum is in its own directory and namespace as there's a standalone
spec for it, that will later also house AOs.

- https://w3c.github.io/webappsec-referrer-policy/
- https://www.w3.org/TR/referrer-policy/

Userland/Libraries/LibWeb/Forward.h

@@ -357,6 +357,10 @@ namespace Web::Platform {
 class Timer;
 }
 
+namespace Web::ReferrerPolicy {
+enum class ReferrerPolicy;
+}
+
 namespace Web::RequestIdleCallback {
 class IdleDeadline;
 }