refactor the runtime code

重构了运行时代码，支持全新的文件系统。

Assets/YooAsset/Runtime/FileSystem/EFileAccess.cs

@@ -0,0 +1,21 @@
+
+namespace YooAsset
+{
+    internal enum EFileAccess 
+    {
+        /// <summary>
+        /// 只可读
+        /// </summary>
+        Read = 0,
+
+        /// <summary>
+        /// 只可写
+        /// </summary>
+        Write = 1,
+
+        /// <summary>
+        /// 可读写
+        /// </summary>
+        ReadWrite = 2
+    }
+}