Describe the bug
We can easily copy fields from the root of the input object into our output by using { field } syntax, but there is no way to copy nested fields with the same shorthand. I can't say { fields.field }, I must use the cumbersome { fields: { field: .fields.field } }.

This is especially tedious when your desired field is many layers deep and you want to keep it in context.

While I appreciate there can be questions around what the expected output is when navigating nested objects, I think in this case the same short hand logic makes unambiguous sense - it is still "create a field called this, with the contents of this", it's just that now this has multiple layers to it.

To Reproduce

$ echo '{"a":{"b":1}}' | jq '{ a }'
{
  "a": {
    "b": 1
  }
}
$ echo '{"a":{"b":1}}' | jq '{ a.b }'
jq: error: syntax error, unexpected FIELD, expecting '}' (Unix shell quoting issues?) at <top-level>, line 1:
{a.b}  
jq: 1 compile error
$

Expected behavior
Output should include the value of the selected field

$ echo '{"a":{"b":1,"junk":0},"useless":0}' | jq '{ a.b }'
{
  "a": {
    "b": 1
  }
}
$

Environment (please complete the following information):
jq-1.6

Additional context
Add any other context about the problem here.