fixes 'std::ptrdiff_t' to 'std::size_t' casting error

addresses this compiler warning:

```text
[..]/magic_enum_utility.hpp:101:31: warning: conversion to ‘std::size_t’ {aka ‘long unsigned int’} from ‘std::ptrdiff_t’ {aka ‘long int’} may change the sign of the result [-Wsign-conversion]
  101 |       return enum_value<D, S>(index);
      |                               ^~~~~
```

include/magic_enum_utility.hpp

@@ -84,7 +84,7 @@ template <typename E, detail::enum_subtype S = detail::subtype_v<E>>
   if (const auto i = enum_index<D, S>(value)) {
     const std::ptrdiff_t index = (static_cast<std::ptrdiff_t>(*i) + n);
     if (index >= 0 && index < count) {
-      return enum_value<D, S>(index);
+      return enum_value<D, S>(static_cast<std::size_t>(index));
     }
   }
   return {};
@@ -98,7 +98,7 @@ template <typename E, detail::enum_subtype S = detail::subtype_v<E>>
   if (const auto i = enum_index<D, S>(value)) {
     const std::ptrdiff_t index = ((((static_cast<std::ptrdiff_t>(*i) + n) % count) + count) % count);
     if (index >= 0 && index < count) {
-      return enum_value<D, S>(index);
+      return enum_value<D, S>(static_cast<std::size_t>(index));
     }
   }
   return MAGIC_ENUM_ASSERT(false), value;
@@ -112,7 +112,7 @@ template <typename E, detail::enum_subtype S = detail::subtype_v<E>>
   if (const auto i = enum_index<D, S>(value)) {
     const std::ptrdiff_t index = (static_cast<std::ptrdiff_t>(*i) - n);
     if (index >= 0 && index < count) {
-      return enum_value<D, S>(index);
+      return enum_value<D, S>(static_cast<std::size_t>(index));
     }
   }
   return {};
@@ -126,7 +126,7 @@ template <typename E, detail::enum_subtype S = detail::subtype_v<E>>
   if (const auto i = enum_index<D, S>(value)) {
     const std::ptrdiff_t index = ((((static_cast<std::ptrdiff_t>(*i) - n) % count) + count) % count);
     if (index >= 0 && index < count) {
-      return enum_value<D, S>(index);
+      return enum_value<D, S>(static_cast<std::size_t>(index));
     }
   }
   return MAGIC_ENUM_ASSERT(false), value;