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/Libraries/LibJS/Parser.h

@@ -184,7 +184,7 @@ public:
                 if (!hint.is_empty())
                     warnln("{}", hint);
             }
-            warnln("SyntaxError: {}", error.to_string());
+            warnln("SyntaxError: {}", error.to_deprecated_string());
         }
     }