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/SourceSnapshotParams.h

@@ -17,7 +17,7 @@ struct SourceSnapshotParams {
     bool has_transient_activation;
 
     // a sandboxing flag set
-    SandboxingFlagSet sandboxing_flags;
+    SandboxingFlagSet sandboxing_flags = {};
 
     // a boolean
     bool allows_downloading;