* Rework VFS to support creating and writing files and introduce OsFileSystem
OsFileSystem abstracts a directory on the device using the filesystem API.
This also introduces GetEntryType and changes FileExists to use it.
* Implement the Horizon FileSystem APIs using our VFS framework
Horizon provides access to files through its IFileSystem class, we can
closely map this to our vfs::FileSystem class.
* Add support for creating application savedata
This implements basic savedata creation using the OsFileSystem API. The
data is stored in Skyline's private directory is stored in the same
format as yuzu.
