AK+Everywhere: Rename String to DeprecatedString

We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)

Userland/Libraries/LibJS/Runtime/Reference.cpp

@@ -194,7 +194,7 @@ ThrowCompletionOr<bool> Reference::delete_(VM& vm)
     return m_base_environment->delete_binding(vm, m_name.as_string());
 }
 
-String Reference::to_string() const
+DeprecatedString Reference::to_string() const
 {
     StringBuilder builder;
     builder.append("Reference { Base="sv);