LibWeb: Introduce the Environment Settings Object

The environment settings object is effectively the context a piece of
script is running under, for example, it contains the origin,
responsible document, realm, global object and event loop for the
current context. This effectively replaces ScriptExecutionContext, but
it cannot be removed in this commit as EventTarget still depends on it.

https://html.spec.whatwg.org/multipage/webappapis.html#environment-settings-object

Userland/Libraries/LibJS/Runtime/ExecutionContext.h

@@ -10,6 +10,7 @@
 #include <AK/FlyString.h>
 #include <AK/WeakPtr.h>
 #include <LibJS/Forward.h>
+#include <LibJS/Module.h>
 #include <LibJS/Runtime/MarkedValueList.h>
 #include <LibJS/Runtime/PrivateEnvironment.h>
 #include <LibJS/Runtime/Value.h>
@@ -62,6 +63,10 @@ public:
     Value this_value;
     MarkedValueList arguments;
     bool is_strict_mode { false };
+
+    // https://html.spec.whatwg.org/multipage/webappapis.html#skip-when-determining-incumbent-counter
+    // FIXME: Move this out of LibJS (e.g. by using the CustomData concept), as it's used exclusively by LibWeb.
+    size_t skip_when_determining_incumbent_counter { 0 };
 };
 
 }