Make CommandLineArgs parsers more generic

[dninemfive]

Currently, there is a lot of duplicate code in parsers for slightly different operations, most notably parsing all args to a type and parsing the first arg to the same type. This can likely be simplified by making the base parser signature IEnumerable<T?> Parser<T>(IEnumerable<string>? values, bool flag) and adding generic extension methods for things like .FirstValid().