Fix hash parsing with Haml 6

[ledermann]

This PR fixes #35 which describes a bug when formatting a multi-line hash as in this (correctly formatted) example:

%ul.nav.nav-tabs.mb-4
  %li.nav-item
    %button.nav-link.active{
      data: { bs_toggle: "tab", bs_target: "#file" },
      type: "button"
    }
      File
  %li.nav-item
    %button.nav-link{
      data: { bs_toggle: "tab", bs_target: "#video" },
      type: "button"
    }
      Video

With Haml 5, there is no problem. But with Haml 6, the indention of the hash is removed because Haml::AttributeParser seems (?!) not to allow line breaks while parsing. This results in the hash being treated as a PlainPart (no indention) rather than a HashAttributesPart (with indention).

Maybe this is a bug in Haml 6 (I haven't investigated this further), but to fix this here, the line breaks will be removed before parsing.

This PR also adds a test that fails without the change and passes with the change.

[Petercopter]

@ledermann This works for me, thank you! 👍

[kddnewton]

This is now fixed in version 4. It strips out our use of the HAML parser entirely and just uses syntax tree now. Thanks for the contribution and report!