LibJS: Shrink DeclarativeEnvironment bindings vector to fit

After setting up all the bindings in function_declaration_instantiation,
we now ask DeclarativeEnvironment to do a shrink_to_fit() on its vector
of bindings.

This ends up saving 5.6 MiB on twitter.com/awesomekling :^)

Userland/Libraries/LibJS/Runtime/DeclarativeEnvironment.cpp

@@ -231,4 +231,9 @@ void DeclarativeEnvironment::initialize_or_set_mutable_binding(Badge<ScopeNode>,
     MUST(initialize_or_set_mutable_binding(vm, name, value));
 }
 
+void DeclarativeEnvironment::shrink_to_fit()
+{
+    m_bindings.shrink_to_fit();
+}
+
 }