[Syntax] Make `,` optional for multiple receiver `-> [...]` with newlines `\n` (?)

Originally it was made this to be consistent with the `list` literals syntax but many times I look at this and think it's not needed

```
-> [
  x,
  y,
  z
]
```

Make it (**when multiple lines**)

```
-> [
  x
  y
  z
]
```