Fix conversion a surrogate pair

[phprus]

Fix for issue: #4094

Errors without fix:
https://github.com/phprus/fmt/actions/runs/10160437906/job/28096817999

14/19 Test #14: std-test .........................***Failed    0.02 sec
[==========] Running 18 tests from 2 test suites.
[----------] Global test environment set-up.
[----------] 16 tests from std_test
[ RUN      ] std_test.path
D:\a\fmt\fmt\test\std-test.cc(39): error: Expected equality of these values:
  fmt::format("{}", path(L"HEAD \xD83D\xDE00 TAIL"))
    Which is: "HEAD  TAIL"
  "HEAD \xF0\x9F\x98\x80 TAIL"
    Which is: "HEAD \xF0\x9F\x98\x80 TAIL"
    As Text: "HEAD 😀 TAIL"
D:\a\fmt\fmt\test\std-test.cc(40): error: Expected equality of these values:
  fmt::format("{}", path(L"HEAD \xD83D\xD83D\xDE00 TAIL"))
    Which is: "HEAD \xEF\xBF\xBD TAIL"
    As Text: "HEAD � TAIL"
  "HEAD �\xF0\x9F\x98\x80 TAIL"
    Which is: "HEAD \xEF\xBF\xBD\xF0\x9F\x98\x80 TAIL"
    As Text: "HEAD �😀 TAIL"
[  FAILED  ] std_test.path (1 ms)

[phprus]

This bug was added on commit:
3dec65b

[vitaut]

Thanks for the fix!