LibJS: Rename ExecutionContext::callee => function

This matches what ECMAScript calls it. Also make it a JS::Function*
instead of a generic Value, since it will always either be a function
object or null.

Userland/Libraries/LibJS/Runtime/VM.h

@@ -45,7 +45,7 @@ struct ScopeFrame {
 struct ExecutionContext {
     const ASTNode* current_node { nullptr };
     FlyString function_name;
-    Value callee;
+    Function* function { nullptr };
     Value this_value;
     Vector<Value> arguments;
     Array* arguments_object { nullptr };