Support both generic and native format of std::filesystem::path

[zhihaoy]

Why

Need a way to include the paths with only slashes rather than backslashes in the output in a cross-platform manner. This can be done by introducing type in format-spec for path.

How to use the proposed feature

On Windows,

std::filesystem::path filename = R"(C:\Users\zhihaoy\.cache)";
print("|{}|", filename);  // prints |C:\Users\zhihaoy\.cache|
print("|{:n}|", filename);  // prints `.native()` |C:\Users\zhihaoy\.cache|
print("|{:g}|", filename);  // prints `.generic_wstring()` |C:/Users/zhihaoy/.cache|

On POSIX, the last line prints .generic_string().

[vitaut]

Sounds reasonable. A PR would be welcome!