New block: Chapter List

[ryelle]

This ports the "Chapter list" block from Developer to the mu-plugins so it can be used on other handbook sites (Five for the Future, in this case).

The only change to the code was setting up some default CSS, so the styles in the child theme.json can be removed.

See WordPress/wporg-developer#539

Screenshots

There should be no visual changes.

To test

1. Apply this PR, build files
2. Apply the wporg-developer PR Blocks: Remove Chapter List wporg-developer#539
3. OR, test with Five for the Future, if you have handbook content set up locally
4. View a handbook page
5. The "Chapter list" left sidebar should appear
6. It should work like production— a list of page links, with the current item bold
7. It should scroll with the page content, unless it's longer than the footer in which case it will scroll internally

[adamwoodnz]

Tested on local developer site and LGTM.

Haven't tested 5ftf.