We would like to move Parley to a design where the "builders" (such as RangedBuilder and TreeBuilder) are external, built on top of a core API that is agnostic to the high-level representation (the core will likely represent styles as a set of non-overlapping ranges).

In order to do this we need to expose more of the low-level parts of Parley such as LayoutContext and possibly ResolveContext, BidiResolver and ShapeContext.

Making changes such that the existing builders depend only on public APIs is likely to a good way to start.