xp::stringOf() for var[]

This pull request suggest rendering var[] as `[x, y, z]` instead of the multi-line output including keys. The detection is done consistent with `typeof()` as it checks the first array key if it's `0`.

## Old behavior
This creates a bloated version

```sh
$ xp -w '[1, 2, 3]'
[
  0 => 1
  1 => 2
  2 => 3
]
```

## New behavior
Arrays are now rendered much like their syntactic input:

```sh
$ xp -w '[1, 2, 3]'
[1, 2, 3]
```

## Behavior for [:var]
This is unchanged:

```sh
$ xp -w '["color" => "green"]'
[
  color => "green"
]
```