Remove custom platform implementation of `DeleteFile` and replace single
use with `std::filesystem::remove` after a negative
`std::filesystem::is_directory` check.
Remove custom platform implementation of `DeleteFolder` and replace two
uses with `std::filesystem::remove_all` which removes a file or
directory with all its contents.
C++17ification!
- Filesystem interaction now uses std::filesystem::path.
- Usage of const char*, std::string have been changed to
std::string_view where appropriate.
- Usage of printf-style functions changed to use fmt.
In the 'OpenExisting' function, 'open_access' is used to create the flags for opening the file.
However, it is not set to zero, so could end up being incorrect file modes, potentially even emptying the file for appending, which would be bad.
This simply initialises it to zero first.
