Specifying spine vertebrae in a more structured format

[breezykermo]

Spines are currently specified through a list of glob strings:

[epub.spine]
title = "Dastardly reason"
vertebrae = ["chapter1.typ", "chapter2.typ", "*.typ"]

This doesn't allow any sense of structure, however: at best you can have an ordered 'flat' list of chapters. What we really want is something more link mdbook's SUMMARY.md, where the hierarchy can be more complex.

One option would be to use TOML array of tables:

[[vertebra]]
title = "Introduction"
path = "intro.typ"

[[vertebra]]
title = "Getting Started"
path = "getting-started.typ"

  [[vertebra.sections]]
  title = "Installation"
  path = "installation.typ"

  [[vertebra.sections]]
  title = "Configuration"
  path = "configuration.typ"

[[vertebra]]
title = "Advanced Topics"
path = "advanced.typ"