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/Promise.h

@@ -45,6 +45,8 @@ public:
     Value reject(Value reason);
     Value perform_then(Value on_fulfilled, Value on_rejected, Optional<PromiseCapability> result_capability);
 
+    bool is_handled() const { return m_is_handled; }
+
 protected:
     virtual void visit_edges(Visitor&) override;