Error out when attempting to generate a default for a disjunction with multiple defaults

[sdboyer]

CUE itself falls back to bottom when there's a disjunction with multiple defaults on it. cuetsy should preserve these semantics, erroring out when it is requested that it generate a default const for an object that contains multiple default values.

[ying-jeanne]

the current behaviour when testing:

-- cue --
E3: "a" | *"b" | *"c" @cuetsy(kind="enum")

-- err --
export enum E3 {
  A = 'a',
  B = 'b',
  C = 'c',
}
export const e3Default: E3 = E3.B' | 'C