Start work on ANS entropy

Implemented ANS constants

src/ImageSharp/Formats/Jxl/IO/JxlAnsConstants.cs

@@ -0,0 +1,15 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+namespace SixLabors.ImageSharp.Formats.Jxl.IO;
+
+internal static class JxlAnsConstants
+{
+    public const int AnsLogTableSize = 12;
+    public const int AnsTableSize = 1 << AnsLogTableSize;
+    public const int AnsTabMask = AnsTableSize - 1;
+    public const int PrefixMaxAlphabetSize = 4096;
+    public const int AnsMaxAlphabetSize = 256;
+    public const int PrefixMaxBits = 15;
+    public const int AnsSignature = 0x13;
+}