Add and use VPK constants

BurnOutSharp.Models/VPK/Constants.cs

@@ -0,0 +1,21 @@
+namespace BurnOutSharp.Models.VPK
+{
+    public static class Constants
+    {
+        public static readonly byte[] SignatureBytes = new byte[] { 0x34, 0x12, 0xaa, 0x55 };
+
+        public const string SignatureString = "4\u0012\u00aaU";
+
+        public const uint SignatureUInt32 = 0x55aa1234;
+
+        /// <summary>
+        /// Index indicating that there is no archive
+        /// </summary>
+        public const int HL_VPK_NO_ARCHIVE = 0x7fff;
+
+        /// <summary>
+        /// Length of a VPK checksum in bytes
+        /// </summary>
+        public const int HL_VPK_CHECKSUM_LENGTH = 0x00008000;
+    }
+}