+ Remove Text.Pandoc.Pretty; use doclayout instead. [API change]
+ Text.Pandoc.Writers.Shared: remove metaToJSON, metaToJSON'
[API change].
+ Text.Pandoc.Writers.Shared: modify `addVariablesToContext`,
`defField`, `setField`, `getField`, `resetField` to work with
Context rather than JSON values. [API change]
+ Text.Pandoc.Writers.Shared: export new function `endsWithPlain` [API
change].
+ Use new templates and doclayout in writers.
+ Use Doc-based templates in all writers.
+ Adjust three tests for minor template rendering differences.
+ Added indentation to body in docbook4, docbook5 templates.
The main impact of this change is better reflowing of content
interpolated into templates. Previously, interpolated variables
were rendered independently and intepolated as strings, which could lead
to overly long lines. Now the templates interpolated as Doc values
which may include breaking spaces, and reflowing occurs
after template interpolation rather than before.
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