[Feature Request] Haml support

[richardmarbach]

We have a codebase that uses Haml as the templating engine. It would be great if packwerk could analyse Haml files as well.

Is Haml support interesting for the project? If there's interest, I'd be willing to contribute the feature.

[gmcgibbon]

Haml support makes sense to me, but it seems like it might make sense as a separate extension gem. You'll have to make a new parser, and likely improve the API for doing this in the parser factory.

[alexevanczuk]

There's a related PR to improve the API for extending parsers here: #243

[richardmarbach]

I've prepared haml support as an extension: https://github.com/richardmarbach/packwerk-haml

It currently depends on the PR branch: #375. Once the parser interface is finalised, I can adjust the haml parser to match it :)