LibJS+LibWeb: Make JS::ExecutionContext protect its Web::HTML::ESO owner

We can't be nuking the ESO while its owned execution context is still on
the VM's execution context stack, as that may lead to a use-after-free.

This patch solves this by adding a `context_owner` field to each context
and treating it as a GC root.

Userland/Libraries/LibJS/Runtime/ExecutionContext.h

@@ -58,6 +58,9 @@ public:
     Environment* variable_environment { nullptr };       // [[VariableEnvironment]]
     PrivateEnvironment* private_environment { nullptr }; // [[PrivateEnvironment]]
 
+    // Non-standard: This points at something that owns this ExecutionContext, in case it needs to be protected from GC.
+    Cell* context_owner { nullptr };
+
     ASTNode const* current_node { nullptr };
     FlyString function_name;
     Value this_value;