LibJS: Combine DeclarativeEnvironment's bindings and names into one list

This reduces the size of the DeclarativeEnvironment from 72 bytes to 48
bytes. This savings helps in the context of nested for-loops which use
'let' to bind the initial variable declarations. In this case, the spec
dicates we create a new environment for each loop iteration by way of
the CreatePerIterationEnvironment AO.

In particular, test262's generated RegExp tests contains many loops of
the form:

    for (let i = 0; i < a_number_on_the_order_of_10; ++i)
        for (let j = 0; j < a_number_on_the_order_of_10_thousand; ++j)

This results in creating hundreds of thousands of environments.

Userland/Libraries/LibJS/Runtime/DeclarativeEnvironment.h

@@ -34,7 +34,16 @@ public:
     ThrowCompletionOr<void> initialize_or_set_mutable_binding(GlobalObject& global_object, FlyString const& name, Value value);
 
     // This is not a method defined in the spec! Do not use this in any LibJS (or other spec related) code.
-    [[nodiscard]] Vector<FlyString> const& bindings() const { return m_names; }
+    [[nodiscard]] Vector<FlyString> bindings() const
+    {
+        Vector<FlyString> names;
+        names.ensure_capacity(m_bindings.size());
+
+        for (auto const& binding : m_bindings)
+            names.unchecked_append(binding.name);
+
+        return names;
+    }
 
     ThrowCompletionOr<Value> get_binding_value_direct(GlobalObject&, size_t index, bool strict);
     ThrowCompletionOr<void> set_mutable_binding_direct(GlobalObject&, size_t index, Value, bool strict);
@@ -45,7 +54,19 @@ protected:
 private:
     virtual bool is_declarative_environment() const override { return true; }
 
+    Optional<size_t> find_binding_index(FlyString const& name) const
+    {
+        auto it = m_bindings.find_if([&](auto const& binding) {
+            return binding.name == name;
+        });
+
+        if (it == m_bindings.end())
+            return {};
+        return it.index();
+    }
+
     struct Binding {
+        FlyString name;
         Value value;
         bool strict { false };
         bool mutable_ { false };
@@ -53,7 +74,6 @@ private:
         bool initialized { false };
     };
 
-    Vector<FlyString> m_names;
     Vector<Binding> m_bindings;
 };