Clean up based on .NET Core 3.1 reccomendations

SabreTools.IO/Readers/ClrMameProReader.cs

@@ -125,7 +125,7 @@ namespace SabreTools.IO.Readers
             // Standalone (special case for DC dats)
             if (CurrentLine.StartsWith("Name:"))
             {
-                string temp = CurrentLine.Substring("Name:".Length).Trim();
+                string temp = CurrentLine["Name:".Length..].Trim();
                 CurrentLine = $"Name: {temp}";
             }
 

SabreTools.IO/Writers/ClrMameProWriter.cs

@@ -59,7 +59,7 @@ namespace SabreTools.IO.Writers
         /// <summary>
         /// Internal stream writer
         /// </summary>
-        private StreamWriter sw;
+        private readonly StreamWriter sw;
 
         /// <summary>
         /// Stack for tracking current node

SabreTools.IO/Writers/IniWriter.cs

@@ -9,7 +9,7 @@ namespace SabreTools.IO.Writers
         /// <summary>
         /// Internal stream writer for outputting
         /// </summary>
-        private StreamWriter sw;
+        private readonly StreamWriter sw;
 
         /// <summary>
         /// Constructor for writing to a file

SabreTools.IO/Writers/SeparatedValueWriter.cs

@@ -9,7 +9,7 @@ namespace SabreTools.IO.Writers
         /// <summary>
         /// Internal stream writer for outputting
         /// </summary>
-        private StreamWriter sw;
+        private readonly StreamWriter sw;
 
         /// <summary>
         /// Internal value if we've written a header before