Add support of shorthand property syntax

[ericleib]

This Pull Request adds support for the shorthand property notation.

This allows to simplify an expression from:

{
  "a": a
}

to:

{
  a
}

This also works for variables:

(
  $foo:= 42;
  {$foo}
)

output:

{
  "foo": 42
}

[mattbaileyuk]

I believe it's quite a Javascript-y syntax concept, but one I do find convenient, and the implementation is not tied to Javascript, so I've got nothing against including this 🙂

Thoughts, @andrew-coleman?