nixd: code actions

[inclyc]

• add undefined variable as top-level formal
• split an subexpression, with import ./file
• with ..; let / inherit

[VTimofeenko]

If I may request, a couple more:

• flatten attrset:

  { foo = { bar = "foz"; } };

  to

  { foo.bar = "foz"; }

• pack into nested attrset:

  foo.bar = "foz";
  foo.baz = "faz";

  to

  foo = { 
    bar = "foz";
    baz = "faz";
  };

[inclyc]

Thanks, any more ideas are welcomed!