Fix to_json for enums when the enum has an unsigned underlying type. (#4237)

* Enhance the UDT unit test to expose the issue

Add a new enum type with uint64_t as the underlying type.
Use it in the overall UDT. Not strictly needed, but it helps exercise its expected usage.
Create an object of this enum type with a large value (negative if cast to int64_t).
Perform several checks on this object as converted to `json`, which fail without the fix.

* Fix the issue in the relevant `to_json` overload.

Select the correct json type depending on the signedness of the enum's underlying type.
This fixes the new checks in the unit test.

* Add the fix to the single_include

I ran `make pretty` but that modified 20 files, performing a significant amount of indentation changes, none of them related to my change.
I ran `make amalgamate`, but that did nothing. Apparently, the make rule won't run if the single_include files have already been updated by `make pretty`.
I forced `make amalgamate` to do the work by touching the file with the fix.
I then decided to keep just the minimal needed change: the addition of the fix to the single_include file.

I just am not conversant enough in Linux to know whether I installed astyle correctly (had to clone the source from a beta branch and build, in order to get support for `--squeeze-lines`).

* Resolve CI errors and use qualified `std::uint64_t`

The fix was relying on implicit conversions in the non-taken branch.
- Ordinarily (work on a C++20 codebase) I would have used `if constexpr` here, sidestepping the issue, but that's not available on C++11 so I didn't bother.
- So instead of an `if` statement, I used a compile-time constant to select the correct overload.
- This is arguably better in this case, anyway.

I was using function-style casts for typed constants, which I consider superior for constants, but the CI checks disagree, so changed all to `static_cast`.
- For some reason, the CI checks didn't point at all of them, so I hope I caught them all myself.

Built with clang14 and all unit tests pass.

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Co-authored-by: Juan Carlos Arevalo Baeza (JCAB) <jcab@ntdev.microsoft.com>

single_include/nlohmann/json.hpp

@@ -5697,7 +5697,8 @@ template<typename BasicJsonType, typename EnumType,
 inline void to_json(BasicJsonType& j, EnumType e) noexcept
 {
     using underlying_type = typename std::underlying_type<EnumType>::type;
-    external_constructor<value_t::number_integer>::construct(j, static_cast<underlying_type>(e));
+    static constexpr value_t integral_value_t = std::is_unsigned<underlying_type>::value ? value_t::number_unsigned : value_t::number_integer;
+    external_constructor<integral_value_t>::construct(j, static_cast<underlying_type>(e));
 }
 #endif  // JSON_DISABLE_ENUM_SERIALIZATION