Issue #721

-- in.cue --
// allow front-style commas
a
: { "key": "value"
, "key2"
: "value2"
}

, b: [
    0
,   1
,       2
        ,3,
    4

    , 5
]
-- out/compile --
--- in.cue
{
  a: {
    key: "value"
    key2: "value2"
  }
  b: [
    0,
    1,
    2,
    3,
    4,
    5,
  ]
}
-- out/eval --
(struct){
  a: (struct){
    key: (string){ "value" }
    key2: (string){ "value2" }
  }
  b: (#list){
    0: (int){ 0 }
    1: (int){ 1 }
    2: (int){ 2 }
    3: (int){ 3 }
    4: (int){ 4 }
    5: (int){ 5 }
  }
}