Everywhere: Rename to_{string => deprecated_string}() where applicable

This will make it easier to support both string types at the same time
while we convert code, and tracking down remaining uses.

One big exception is Value::to_string() in LibJS, where the name is
dictated by the ToString AO.

Userland/Utilities/copy.cpp

@@ -51,7 +51,7 @@ static ErrorOr<Options> parse_options(Main::Arguments arguments)
         // Copy the rest of our command-line args.
         StringBuilder builder;
         builder.join(' ', text);
-        options.data = builder.to_string();
+        options.data = builder.to_deprecated_string();
     }
 
     return options;