The "default" option is no longer represented as `Nothing` but via a new
type constructor, making the `Maybe` wrapper superfluous.
The default behavior of using heuristics can now be enabled explicitly
by setting `--top-level-division=default`.
API change (`Text.Pandoc.Options`): The `Division` type was renamed to
`TopLevelDivision`. The `Section`, `Chapter`, and `Part` constructors
were renamed to `TopLevelSection`, `TopLevelChapter`, and
`TopLevelPart`, respectively. An additional `TopLevelDefault`
constructor was added, which is now also the new default value of the
`writerTopLevelDivision` field in `WriterOptions`.
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