feat: add decoder Computation Expression

[njlr]

This PR adds a Computation Expression for decoding.

It is called decoder and works as one might expect:

type Item =
    | Book of title : string * year : int * stars : int
    | Author of fullName : string * stars : int

module Item =

    let decode : Decoder<Item> =
        decoder {
            let! title = Decode.optional "title" Decode.string
            let! fullName = Decode.optional "fullName" Decode.string
            let! stars = Decode.field "stars" Decode.int

            match title, fullName with
            | Some title, None ->
                let! year = Decode.field "year" Decode.int

                return Book (title, year, stars)
            | None, Some fullName ->
                return Author (fullName, stars)
            | Some _, Some _ ->
                return! Decode.fail $"Must not specify `title` and `fullName` properties"
            | None, None ->
                return! Decode.fail $"Must specify either a `title` or `fullName` property"
        }

The primary use-case is complex conditional decoders where Decode.andThen is typically used. Elm lacks CEs so has to lean heavily on andThen, but we don't have this limitation! 😄

Related: #121