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/Forward.h

@@ -457,7 +457,7 @@ struct EnvironmentSettingsObject;
 struct NavigationParams;
 struct POSTResource;
 struct PolicyContainer;
-struct SandboxingFlagSet;
+enum class SandboxingFlagSet;
 struct SerializedFormData;
 struct SessionHistoryEntry;
 }