To support experimentation with ICU4X backed segmentation, we'll need to move the LineBreaker and WordBreaker implementations into intl/components to make them available to standalone SpiderMonkey builds.
Note that there's a function NS_GetComplexLineBreaks  that is currently implemented differently for each platform. I don't think we want to unify that. I'm hoping we can use a forward declaration and let the linker find it at link time. We'd then define a
no-op implementation for SpiderMonkey builds. The result of this would be that the messy platform specific bits still live in lwbrk, but we've unified the interfaces into intl/components.
We might also want to clean up the API a bit while doing this, e.g. renaming GetJISx4051Breaks to something more generic, and making sure the WordBreaker API is a good match for Intl.Segmenter, but this could also be done through follow ups.
For reference, here is Makoto's work integrating ICU4X as a Line Breaker: https://treeherder.mozilla.org/jobs?repo=try&revision=34ea1b318db5109891dac88f8adf479e144ec9dd