59. implement MapStringUnionToObjectUnion<U>

  - accepted / - tried

Implement MapStringUnionToObjectUnion<U> to map a string union to an object union as below.

type A = MapStringUnionToObjectUnion<'1'>
// {
//   value: '1'
// } 

type B = MapStringUnionToObjectUnion<'1' | '2'>
// {
//   value: '1'
// } |
// {
//   value: '2
// }

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