FoundationEssentials: fix corner case of flushing file descriptor (#723)

On Windows, `_commit` can only be used for file descriptors backing
files that are on disks. We would previously flush all file types which
would cause an error.

Thanks to Jeremy Day for tracking down the cause of the error!

Author: compnerd

Sources/FoundationEssentials/Data/Data+Writing.swift

@@ -230,7 +230,14 @@ private func write(buffer: UnsafeRawBufferPointer, toFileDescriptor fd: Int32, p
 
     if !buffer.isEmpty {
 #if os(Windows)
-        let res = _commit(fd)
+        let hFile: HANDLE? = HANDLE(bitPattern: _get_osfhandle(fd))
+        // On Windows, only call _commit if the fd corresponds to an actual file
+        // on disk.
+        let res: CInt = if let hFile, GetFileType(hFile) == FILE_TYPE_DISK {
+            _commit(fd)
+        } else {
+            0
+        }
 #else
         let res = fsync(fd)
 #endif