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.

Tests/LibSQL/TestSqlExpressionParser.cpp

@@ -39,7 +39,7 @@ ParseResult parse(StringView sql)
     auto expression = parser.parse();
 
     if (parser.has_errors()) {
-        return parser.errors()[0].to_string();
+        return parser.errors()[0].to_deprecated_string();
     }
 
     return expression;