file_util: Migrate remaining file handling functions over to std::filesystem

Converts creation and deletion functions over to std::filesystem,
simplifying our file-handling code.

Notably with this, CopyDir will now function on Windows.

src/core/file_sys/savedata_archive.cpp

@@ -200,7 +200,7 @@ static ResultCode DeleteDirectoryHelper(const Path& path, const std::string& mou
 }
 
 ResultCode SaveDataArchive::DeleteDirectory(const Path& path) const {
-    return DeleteDirectoryHelper(path, mount_point, Common::FS::DeleteDir);
+    return DeleteDirectoryHelper(path, mount_point, Common::FS::Delete);
 }
 
 ResultCode SaveDataArchive::DeleteDirectoryRecursively(const Path& path) const {