feat: jsonnetfmt remove newlines after ObjectField id

[DinoChiesa]

add handling of ObjectField in fix_newlines.go

With this input:

local c =
  import "json2xml.libjsonnet";

local d = {
  name:

    "foo",

  children:
 ["bar", "bam"],
};

{
  output:
  c.manifestXml(d, "elements"),
}

BEFORE this change:

local c =
  import 'json2xml.libjsonnet';

local d = {
  name:

    'foo',

  children:
    ['bar', 'bam'],
};

{
  output:
    c.manifestXml(d, 'elements'),
}

WITH this change:

local c =
  import 'json2xml.libjsonnet';

local d = {
  name: 'foo',

  children: ['bar', 'bam'],
};

{
  output: c.manifestXml(d, 'elements'),
}

[rudo-thomas]

FWIW, I think the formatter works as I would expect already and it is consistent: it doesn't remove newlines in the syntax described in this PR, neither in other constructs – local variables after the =, after function parameters, ...