LibWeb: Move Fetch infra into the Web::Fetch::Infrastructure namespace

The Fetch spec unfortunately will cause a name clash between the Request
concept and the Request JS object - both cannot live in the Web::Fetch
namespace, and WrapperGenerator generally assumes `Web::<Name>` for
things living in the `<Name>/` subdirectory, so let's instead move infra
code into its own namespace - it already sits in a (sub-)subdirectory
anyway.

Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Methods.h

@@ -8,7 +8,7 @@
 
 #include <AK/Forward.h>
 
-namespace Web::Fetch {
+namespace Web::Fetch::Infrastructure {
 
 [[nodiscard]] bool is_method(ReadonlyBytes);
 [[nodiscard]] bool is_cors_safelisted_method(ReadonlyBytes);