Deprecate addin

[pascalberger]

The goal of this addin is to add FIGlet font support to Cake. Since we're now shipping Spectre.Console as part of Cake with 1.0, Cake comes with out-of-the-box FIGlet support:

AnsiConsole.Render(
    new FigletText("Hello")
        .LeftAligned()
        .Color(Color.Red));

See also cake-build/cake#3135 for providing alias for it in Cake.Common.