fix: replace uses of deprecated has_formatter

use is_formattable instead.

has_formatter was never a good API, since it only asked if the type has
a `formatter` specialization, but a type can also be formattable if
it provides an implicit conversion to a type which does.

The newer API takes care of the that problem.

Author: mizvekov

include/mrdocs/Support/Handlebars.hpp

@@ -218,7 +218,7 @@ class MRDOCS_DECL OutputRef
         @return A reference to this object
      */
     template <class T>
-    requires fmt::has_formatter<T, fmt::format_context>::value
+    requires fmt::is_formattable<T, fmt::format_context>::value
     friend
     OutputRef&
     operator<<( OutputRef& os, T v )