Implement basic filesystem support in VFS

This mirrors Horizon's IFileSystem, it will be used by the nsp and nca
loaders to read their content.
https://switchbrew.org/wiki/Filesystem_services#IFileSystem

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

@@ -0,0 +1,48 @@
+// SPDX-License-Identifier: MPL-2.0
+// Copyright © 2020 Skyline Team and Contributors (https://github.com/skyline-emu/)
+
+#pragma once
+
+#include <common.h>
+#include "backing.h"
+#include "directory.h"
+
+namespace skyline::vfs {
+    /**
+     * @brief The FileSystem class represents an abstract filesystem with child files and folders
+     */
+    class FileSystem {
+      public:
+        FileSystem() = default;
+
+        /* Delete the move constructor to prevent multiple instances of the same filesystem */
+        FileSystem(const FileSystem &) = delete;
+
+        FileSystem &operator=(const FileSystem &) = delete;
+
+        virtual ~FileSystem() = default;
+
+        /**
+         * @brief Opens a file from the specified path in the filesystem
+         * @param path The path to the file
+         * @param mode The mode to open the file with
+         * @return A shared pointer to a Backing object of the file
+         */
+        virtual std::shared_ptr<Backing> OpenFile(std::string path, Backing::Mode mode = {true, false, false}) = 0;
+
+        /**
+         * @brief Checks if a given file exists in a filesystem
+         * @param path The path to the file
+         * @return A boolean containing whether the file exists
+         */
+        virtual bool FileExists(std::string path) = 0;
+
+        /**
+         * @brief Opens a directory from the specified path in the filesystem
+         * @param path The path to the directory
+         * @param listMode The list mode for the directory
+         * @return A shared pointer to a Directory object of the directory
+         */
+        virtual std::shared_ptr<Directory> OpenDirectory(std::string path, Directory::ListMode listMode) = 0;
+    };
+}