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/LibGUI/CommandPalette.cpp

@@ -129,7 +129,7 @@ public:
         case Column::Shortcut:
             if (!action.shortcut().is_valid())
                 return "";
-            return action.shortcut().to_string();
+            return action.shortcut().to_deprecated_string();
         }
 
         VERIFY_NOT_REACHED();