Chapter Navigation Block - move from Prepress to BU Blocks and generalize for all BU Sites

[acketon]

After some discussion with @crystalms I believe the building the Chapter Navigation block in Prepress is not the best location for this block.

Back during the One Editorial project it was nearly finished (90% ??) and was in good shape. We should finish the block but I now believe we should move it here to BU Blocks where it can be used in a wider variety of post types and use cases. We've had a few client projects where TOC has been implemented with either JS, shortcodes, or custom markup and a Block to handle this would be useful for both long editorial content and long standard content on pages.

For example Global Programs will be publishing some long-form "guides" related to global travel, research, etc and we need to implement a Chapter Navigation component. In that project it will probably be a shortcode as it is not a block based project but in the future this block makes more sense here in BU Blocks in our opinion.

https://github.com/bu-ist/bu-prepress/pull/426