fix: mark fmt::streamed() as constexpr

Because it's just performing a very basic type conversion that can be
done at constexpr time.

My use case simultaneously creates a
`fmt::basic_format_string<some_type_conversion<Args...>>` instance and
performs `some_type_conversion<Args>(args)...`. `some_type_conversion`
optionally applies `fmt::streamed(arg)` to a subset of types. This needs
to be `constexpr` because `basic_format_string`'s constructor is
`consteval`.

include/fmt/ostream.h

@@ -140,7 +140,7 @@ struct formatter<detail::streamed_view<T>, Char>
   \endrst
  */
 template <typename T>
-auto streamed(const T& value) -> detail::streamed_view<T> {
+constexpr auto streamed(const T& value) -> detail::streamed_view<T> {
   return {value};
 }