Read a std::u8string from a JSON string and store it as one in every configuration

In C++20, json(std::u8string) was stored as a JSON string only when
std::filesystem::path support was available (the to_json overload sat
inside the JSON_HAS_FILESYSTEM guard), and there was no matching
from_json, so get<std::u8string>() threw type_error.302 ("type must be
array, but is string"). Without std::filesystem, the same expression
still produced an array of code units (the 3.12.0 behavior).

Move the to_json overload out of the filesystem guard so that a
std::u8string is a string in every configuration, build the value
directly from the code units (this also works for a custom string_t
such as the tests' alt_string), and add a from_json overload that reads
a JSON string. The array of numbers written by 3.12.0 is still accepted;
other types throw type_error.302 ("type must be string or array").
std::u8string_view and char8_t arrays are unchanged.

The tests now run in C++20/23 (the file mentions JSON_HAS_CPP_20/23),
also without std::filesystem (test-conversions2_no_filesystem), and with
ordered_json and a custom string type. The new behavior is documented.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/CMakeLists.txt

@@ -364,6 +364,16 @@ json_test_add_test_for(src/unit-class_parser.cpp
     MAIN test_main CXX_STANDARDS ${test_cxx_standards} ${test_force}
 )
 
+# test the conversions again without std::filesystem::path support: a
+# std::u8string must be a JSON string in this configuration, too
+json_test_set_test_options(test-conversions2_no_filesystem
+    COMPILE_DEFINITIONS JSON_HAS_FILESYSTEM=0 JSON_HAS_EXPERIMENTAL_FILESYSTEM=0
+)
+json_test_add_test_for(src/unit-conversions2.cpp
+    NAME test-conversions2_no_filesystem
+    MAIN test_main CXX_STANDARDS ${test_cxx_standards} ${test_force}
+)
+
 # test diagnostic positions again without regular diagnostics (JSON pointer paths)
 json_test_set_test_options(test-diagnostic-positions_only
     COMPILE_DEFINITIONS JSON_DIAGNOSTICS=0