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/cpp-lexer.cpp

@@ -23,7 +23,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
 
     Cpp::Lexer lexer(content);
     lexer.lex_iterable([](auto token) {
-        outln("{}", token.to_string());
+        outln("{}", token.to_deprecated_string());
     });
 
     return 0;