literals operator _json, _json_pointer for char8_t in C++20

[esoft-com]

u8"xxx" is char8_t since C++20，so u8"xxx"_json and u8"xxx"_json_pointer is not work for C++20

[nlohmann]

This affects the same code as #4956, so I should address this in #4957 too.

[nlohmann]

Any ideas?

[gregmarr]

Do you know what it is stored as?
I don't see anything at these two pages that says that they're not stored as UTF-8.
https://learn.microsoft.com/en-us/cpp/cpp/string-and-character-literals-cpp?view=msvc-160
https://learn.microsoft.com/en-us/cpp/cpp/string-and-character-literals-cpp?view=msvc-170

[nlohmann]

These were my changes: 1ab4f7c

This is the failing test: https://github.com/nlohmann/json/actions/runs/18713429662/job/53367039708?pr=4957

However, maybe we don't need to care, because we already state that we require UTF-8.

[gregmarr]

Yeah, I think if you put non-UTF-8 in a literal labeled as UTF-8, then that is a programmer error, not a library error.

[nlohmann]

Good. Then I will re-add the code in a new PR and need to find a way to exclude the test for the respective MSVC jobs.