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/Print.cpp

@@ -82,7 +82,7 @@ DeprecatedString strip_ansi(StringView format_string)
     }
     if (i < format_string.length())
         builder.append(format_string[i]);
-    return builder.to_string();
+    return builder.to_deprecated_string();
 }
 
 template<typename... Args>