Implement the Sass indented syntax

[stof]

Once #424 is complete, implementing the indented syntax won't be a lot of work (I kept the separation between StylesheetParser and ScssParser on purpose to make it possible to port the SassParser from dart-sass).

[stof]

Once this is done, our FrameworkTest should probably also include the Bulma framework, to be consistent with the list used by dart-sass.

[Bizarrus]

Any updates?

[stof]

The v2 is not yet ready. This will either be part of it or of a following minor version (the new parser is not part of 1.x)