Add sim parsing for Steam2 sim/sid installers (#95)

* Add model for sim file

* Add sim parsing for steam2 installer set

* First round of fixes

SabreTools.Data.Models/Steam2Installer/SimFile.cs

@@ -0,0 +1,31 @@
+namespace SabreTools.Data.Models.Steam2Installer
+{
+    /// <summary>
+    /// .sim file for a .sim/.sid archive
+    /// </summary>
+    /// <remarks>
+    /// While it is not known for sure what .sim stands for, it's assumed to mean Steam Installer Manifest.
+    /// This contains information about the .sid data volumes. It's used for retail installer discs for
+    /// Steam2 (Steam pre-Steam3/SteamPipe). Preceded by GCF (GCF and NCF are still used for actual game
+    /// installs on Steam2, sim/sid just replaced GCF for retail discs. NCF was never directly stored on
+    /// retail discs since sim/sid had already come out). Succeeded by csm/csd for Steam3/SteamPipe.
+    /// Code is based on NickNine's python-based sim/sid extractor at https://pastebin.com/sj6F64XP
+    /// While this was not referenced for any of the code in SabreTools, https://codeberg.org/CYBERDEV/SIDEx
+    /// provides additional documentation if it is needed.
+    /// </remarks>
+    public sealed class SimFile
+    {
+        /// <summary>
+        /// .sim file header
+        /// </summary>
+        public Header Header { get; set; } = new();
+
+        // The string bytes array would go here, but it gets parsed out into FileEntries during deserialization,
+        // so there's no point storing it in the model. Same with the file entries size and count.
+
+        /// <summary>
+        /// Array of file entries
+        /// </summary>
+        public FileEntry[] FileEntries { get; set; } = [];
+    }
+}