adjust js generated enum style

[teamplayer3]

Use the first enum style like in this wasm proposal.

type MyEnum =
  | { tag: 'A' }
  | { tag: 'B', value: number }
  | { tag: 'C', value: [number, number] }
  | { tag: 'D', value: { x: number, y: number }
  ;

It allows writing switch cases:

switch (enum.tag) {
    case 'A': ...; break;
    case 'B': .... 
}