A series of indentation fixes

[aszlig]

This adds tests plus a few fixes for indentation, mainly around let bindings.

For example things like this should now be indented correctly:

with import <nixpkgs> {};

let
  foo = "bar";
in foo

[LnL7]

Did you get anywhere with the other issue?

[aszlig]

@LnL7: Not yet, just woke up :sleepy:

[LnL7]

I'll merge this already since it's already an improvement.

[Mic92]

I think this added a regression:

{
  src = { # now write enter };

}

This will extra indent the line with src for every newline that is inserted by two spaces:

{
    src = {
  };
}

{
      src = {

  };
}

[LnL7]

Are you sure this is after the pr?

[Mic92]

Indeed vim-polyglot needs an update: https://github.com/sheerun/vim-polyglot/blob/master/indent/nix.vim#L57