LibWeb: Add 'is scripting enabled' concept to EnvironmentSettingsObject

This is now the source of truth for 'user enabled/disabled scripting',
but it has to ask the window's page, which actually stores the setting.

Also use this new functionality in two places where it was previously
marked as a FIXME.

Userland/Libraries/LibWeb/HTML/Scripting/Environments.h

@@ -101,6 +101,9 @@ struct EnvironmentSettingsObject
 
     void notify_about_rejected_promises(Badge<EventLoop>);
 
+    bool is_scripting_enabled() const;
+    bool is_scripting_disabled() const;
+
 protected:
     explicit EnvironmentSettingsObject(JS::ExecutionContext& realm_execution_context);