Encode, decode number for variants alias

[mununki]

#62
This PR enables the encoding, decoding the number for variants. Responding to the representation Js.Json.Number(float) in the compiler, only float value can be handled.

@spice
type t = | @spice.as(1.0) One | @spice.as(2.0) Two

let encoded = One->t_encode // 1.0
let decoded = Js.Json.number(1.0)->t_decode // One