Use enum class instead of enum

https://github.com/cginternals/cppfs/blob/e6b03ac2f4d919497acec121dc4df0b69c591257/source/cppfs/include/cppfs/cppfs.h#L16
3 definitions in that file. I'm sure that there are another usages of `enum` in the project

Start with source/cppfs/include/cppfs/cppfs.h at the linked line and inspect the three enum definitions mentioned there. Search the repository for other enum usages, then verify that the relevant declarations consistently use enum class and that the project still builds successfully.

Written by the indexing model from the issue text.