add carry-on types and manyfold object union types

[miroir-framework]

Add carry-on types, usable (among other cases) with heteronomous unions:

{
  type: "object",
  definition: {
    a: string,
    b: { c: number }
  },
  carryOn: {
    d: boolean
  } 
}

shall result in a TS type (and Zod schema) corresponding to:

{ a: string | { d: boolean }, b: { c: number  |  { d: boolean } }  |  { d: boolean } } 

For unions among object types, many discriminators can be found, implying the existence of manyfold object union types, that are union types of objects, in which there are many attributes used as discriminators. This is not semantically relevant, as discriminated unions do not exist as a prime citizen in TS types, being treated as any other union. This is however pragmatically relevant, guiding the use of such an object union type. The possibility of having an array of discriminators on a union type shall thus be considered in that perspective.