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/ECMAScriptFunctionObject.h

@@ -9,6 +9,7 @@
 
 #include <LibJS/AST.h>
 #include <LibJS/Bytecode/Generator.h>
+#include <LibJS/Runtime/ExecutionContext.h>
 #include <LibJS/Runtime/FunctionObject.h>
 
 namespace JS {
@@ -86,6 +87,10 @@ public:
 
     FunctionKind kind() const { return m_kind; }
 
+    // This is used by LibWeb to disassociate event handler attribute callback functions from the nearest script on the call stack.
+    // https://html.spec.whatwg.org/multipage/webappapis.html#getting-the-current-value-of-the-event-handler Step 3.11
+    void set_script_or_module(ScriptOrModule script_or_module) { m_script_or_module = move(script_or_module); }
+
 protected:
     virtual bool is_strict_mode() const final { return m_strict; }