Built-in support for `--no`-style flag args

[baronfel]

Currently flag args are of the form --flag <true|false>. There's a bit of a convention to allow --flag or --no-flag. It would be nice to support this in the API.

In the interim, this can be done with a custom ParseArgument delegate:

#r "nuget: System.CommandLine, 2.0.0-beta3.22114.1"

open System.CommandLine
open System.CommandLine.Parsing

let parseFlag = ParseArgument<bool>(fun r ->
    match r.Parent with
    | :? OptionResult as b ->
        b.Token.Value = "--force"
    | _ ->
        r.ErrorMessage <- "unknown thingy"
        Unchecked.defaultof<bool>
)
let c = Option<bool>([| "--force"; "--no-force" |], parseArgument = parseFlag)
c.Arity <- ArgumentArity.Zero


true = c.Parse("--force").GetValueForOption(c)
false = c.Parse("--no-force").GetValueForOption(c)