Directory separator issue in autoloader

Basic autoloading for main the classes in the core autoloader expect the directory separator to be `/`. This will not work in Windows and the classes are not found. Use the directory separator constant in the path strings to fix this issue.

https://github.com/devgeniem/dustpress/blob/master/dustpress.php?fbclid=IwAR1AL6NkypLr3eU-JwKmFvxdZ3YxdZN1EP0c7jf1Jpjpyp9GlBRf3YbhhL4#L1473