seal public classes which don't expose public ctors (#2081)

Author: adamsitnik

src/System.CommandLine/ParseResult.cs

@@ -15,7 +15,7 @@ namespace System.CommandLine
     /// <summary>
     /// Describes the results of parsing a command line input based on a specific parser configuration.
     /// </summary>
-    public class ParseResult
+    public sealed class ParseResult
     {
         private readonly IReadOnlyList<ParseError> _errors;
         private readonly CommandResult _rootCommandResult;

src/System.CommandLine/Parsing/ParseError.cs

@@ -6,7 +6,7 @@ namespace System.CommandLine.Parsing
     /// <summary>
     /// Describes an error that occurs while parsing command line input.
     /// </summary>
-    public class ParseError
+    public sealed class ParseError
     {
         internal ParseError(
             string message,