Move Invoke* and Parse extension methods to their extended Symbol types

[jonsequitur]

    // All of these should be instance methods.
    public static class CommandExtensions 
    {
        public static int Invoke(this Command command, string[] args, IConsole console = null);
        public static int Invoke(this Command command, string commandLine, IConsole console = null);
        public static Task<int> InvokeAsync(this Command command, string[] args, IConsole console = null);
        public static Task<int> InvokeAsync(this Command command, string commandLine, IConsole console = null);
        // The comments from Argument(Extensions)::Parse also apply here.
        public static ParseResult Parse(this Command command, string[] args);
        public static ParseResult Parse(this Command command, string commandLine);
    }

[KalleOlaviNiemitalo]

CommandExtensions.Parse was moved to Command.Parse in #2062.

CommandExtensions.Invoke and CommandExtensions.InvokeAsync have not been moved, nor did I find an open pull request for moving them.

[adamsitnik]

#2106

fixed by #2106