LibJS: Make DeclarativeEnvironment store bindings in a Vector

The previous storage for DeclarativeEnvironment looked like this:

    HashMap<FlyString, Binding> m_bindings;

This patch changes that to:

    HashMap<FlyString, size_t> m_names;
    Vector<Binding> m_bindings;

The main goal here is to give each binding an index that can ultimately
be cached and used for optimized environment accesses.

Userland/Libraries/LibJS/Runtime/DeclarativeEnvironment.h

@@ -48,7 +48,8 @@ private:
         bool initialized { false };
     };
 
-    HashMap<FlyString, Binding> m_bindings;
+    HashMap<FlyString, size_t> m_names;
+    Vector<Binding> m_bindings;
 };
 
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