Fix error C2668 on Windows with option /std:c++latest (fmtlib#3680)

* Namespace-qualify to avoid ambiguity with std::format_to for format-test.cc

When build fmt with MSVC under option /std:c++latest, it failed due to `error 2668: 'std::format_to': ambiguous call to overloaded function`, so add namespace to qualify the call to format_to to avoid this issue.

test/format-test.cc

@@ -1788,7 +1788,7 @@ FMT_BEGIN_NAMESPACE
 template <> struct formatter<point> : nested_formatter<double> {
   auto format(point p, format_context& ctx) const -> decltype(ctx.out()) {
     return write_padded(ctx, [this, p](auto out) -> decltype(out) {
-      return format_to(out, "({}, {})", nested(p.x), nested(p.y));
+      return fmt::format_to(out, "({}, {})", nested(p.x), nested(p.y));
     });
   }
 };