Escape characters in diffable cs field default values (#386)

* Fix diffable cs failing to write \\uD800 in CharUnicodeInfo

* using System;

* Escape characters in diffable cs field default values

Cpp2IL.Core/OutputFormats/DiffableCsOutputFormat.cs

@@ -183,6 +183,8 @@ private static void AppendField(StringBuilder sb, FieldAnalysisContext field, in
 
             if (defaultValue is string stringDefaultValue)
                 sb.Append('"').Append(stringDefaultValue).Append('"');
+            else if (defaultValue is char charDefaultValue)
+                sb.Append("'\\u").Append(((int)charDefaultValue).ToString("X")).Append("'");
             else
                 sb.Append(defaultValue);
         }