Fix compatibility with chars_format in fast_float version 7

This release changes `chars_format` from an `enum` to an `enum class`
with underlying type `uint64_t`.

https://github.com/fastfloat/fast_float/releases/tag/v7.0.0

src/scn/impl.cpp

@@ -721,12 +721,14 @@ scan_expected<std::ptrdiff_t> fast_float_fallback(impl_init_data<CharT> data,
 struct fast_float_impl_base : impl_base {
     fast_float::chars_format get_flags() const
     {
-        unsigned format_flags{};
+        uint64_t format_flags{};
         if ((m_options & float_reader_base::allow_fixed) != 0) {
-            format_flags |= fast_float::fixed;
+            format_flags |=
+                static_cast<uint64_t>(fast_float::chars_format::fixed);
         }
         if ((m_options & float_reader_base::allow_scientific) != 0) {
-            format_flags |= fast_float::scientific;
+            format_flags |=
+                static_cast<uint64_t>(fast_float::chars_format::scientific);
         }
 
         return static_cast<fast_float::chars_format>(format_flags);