Implement a simple directory system in VFS

This mirrors Horizon's IDirectory:
https://switchbrew.org/wiki/Filesystem_services#IDirectory

app/src/main/cpp/skyline/vfs/directory.h

@@ -0,0 +1,59 @@
+// SPDX-License-Identifier: MPL-2.0
+// Copyright © 2020 Skyline Team and Contributors (https://github.com/skyline-emu/)
+
+#pragma once
+
+#include <common.h>
+
+namespace skyline::vfs {
+    /**
+     * @brief The Directory class represents an abstract directory in a filesystem
+     */
+    class Directory {
+      public:
+        /**
+         * @brief This enumerates the types of entry a directory can contain
+         */
+        enum class EntryType : u32 {
+            Directory = 0x0, //!< The entry is a directory
+            File = 0x1, //!< The entry is a file
+        };
+
+        /**
+         * @brief This hold information about a single directory entry
+         */
+        struct Entry {
+            std::string name; //!< The name of the entry
+            EntryType type; //!< The type of the entry
+        };
+
+        /**
+         * @brief This describes what will be returned when reading a directories contents
+         */
+        union ListMode {
+            struct {
+                bool directory : 1; //!< The directory listing will contain subdirectories
+                bool file : 1; //!< The directory listing will contain files
+            };
+            u32 raw; //!< The raw value of the listing mode
+        };
+        static_assert(sizeof(ListMode) == 0x4);
+
+        ListMode listMode; //!< This describes what this directory will return when it is Read
+
+        Directory(ListMode listMode) : listMode(listMode) {}
+
+        /* Delete the move constructor to prevent multiple instances of the same directory */
+        Directory(const Directory &) = delete;
+
+        Directory &operator=(const Directory &) = delete;
+
+        virtual ~Directory() = default;
+
+        /**
+         * @brief Reads the contents of a directory non-recursively
+         * @return A vector containing each entry
+         */
+        virtual std::vector<Entry> Read() = 0;
+    };
+}