LibWeb: Convert SandboxingFlagSet into a enum class

Instead of having a nested enum within a struct, use the macro
AK_ENUM_BITWISE_OPERATORS to add all the convienent has_flag free
functions and such for ease of use.

Userland/Libraries/LibWeb/HTML/Navigable.cpp

@@ -490,7 +490,7 @@ static WebIDL::ExceptionOr<Optional<NavigationParams>> create_navigation_params_
     JS::GCPtr<Fetch::Infrastructure::FetchController> fetch_controller = nullptr;
 
     // 13. Let finalSandboxFlags be an empty sandboxing flag set.
-    SandboxingFlagSet final_sandbox_flags;
+    SandboxingFlagSet final_sandbox_flags = {};
 
     // 16. Let locationURL be null.
     ErrorOr<Optional<AK::URL>> location_url { OptionalNone {} };