LibCore: Expose file type from DirIterator

Our `find` utility makes use of the `dirent::d_type` field for filtering
results, which `Core::DirIterator` didn't expose. So, now it does. :^)

We now store the name and type of the entry as the "next" value instead
of just the name. The type is exposed as a `DirectoryEntry::Type`
instead of a `DT_FOO` constant, so that we're not tied to posixy
systems, and because proper enums are nice. :^)

Userland/Libraries/LibCore/CMakeLists.txt

@@ -6,6 +6,7 @@ set(SOURCES
     DateTime.cpp
     DeprecatedFile.cpp
     Directory.cpp
+    DirectoryEntry.cpp
     DirIterator.cpp
     ElapsedTimer.cpp
     Event.cpp