🔢 Enable continuous numbering across pages

[fwkoch]

This adds an extra key to numbering fields - continue. If true, the numbering of that type will continue counting from the previous page.

This also adds title/page-level enumeration (which, combined with continue means you can have numbered chapters).

Finally, this cleans up how frontmatter numbering heading_# relates to page heading depths. Now, the user-defined frontmatter levels match the user-defined heading depths, even if transforms have changed the depths in the mdast (for normalization, title lifting, etc).

[agoose77]

What is this blocked on? Can I help in any way?

[rowanc1]

I think it was blocked on requirements as we got a bit confused on how to actually do it!

[fwkoch]

I don't think there is anything critical blocking this - I just stepped away from it and have not come back. The biggest thing I could use some input on is the frontmatter fields:

In numbering on a page, you can now specify start: <number> (if you want to start counting at a specific number) or continue: true (if you want to continue counting from the previous page). However, as these are separate fields, you can specify both, e.g.:

numbering:
  figure:
    start: 3
    continue: true

In that case, it is ambiguous which takes priority. So, I think we should reconsider this frontmatter so this ambiguity is not possible. Maybe that means start: number | 'continue' or something? So you can either specify a starting number or say "continue from previous page." Or, we could just leave start and continue but introduce a validation error if both are defined... Or maybe something else?

Beyond that, this PR just needs some more testing and bug fixing - the continuous numbering still doesn't quite work as expected everywhere.