Fix an odr violation in ranges.h (#2483)

* Add a testcase demonstrating ODR violation in ranges.h

* Fix ODR violation in ranges.h

* Fix comments

Co-authored-by: Pierre-Antoine Lacaze <pa.lacaze@gmail.com>

include/fmt/ranges.h

@@ -437,7 +437,7 @@ auto find_escape(const Char* begin, const Char* end)
   return {begin, nullptr, 0};
 }
 
-auto find_escape(const char* begin, const char* end)
+inline auto find_escape(const char* begin, const char* end)
     -> find_escape_result<char> {
   if (!is_utf8()) return find_escape<char>(begin, end);
   auto result = find_escape_result<char>{end, nullptr, 0};