Add `?//` (Destructuring Alternative Operator) support

[A4-Tacks]

$ jq  -n '{x:1} as [$x] ?// {$x} | $x'
1
$ jaq -n '{x:1} as [$x] ?// {$x} | $x'
Error: expected |
  ╭─[<inline>]
  │
1 │ {x:1} as [$x] ?// {$x} | $x
  ┆               ┬            
  ┆               │            
  ┆               ╰───────────── unexpected token
──╯

[01mf02]

Yes, adding this would be nice, but it's probably quite hard and not really on my roadmap.