`Nothing` means: nothing specified.
`Just []` means: an empty list specified (e.g. in defaults).
Potentially these could lead to different behavior: see #5888.
Instead of `$HOME/.pandoc`, the default user data directory is
now `$XDG_DATA_HOME/pandoc`, where `XDG_DATA_HOME` defaults to
`$HOME/.local/share` but can be overridden by setting the environment
variable.
If this directory is missing, then `$HOME/.pandoc` is searched
instead, for backwards compatibility. However, we recommend
moving local pandoc data files from `$HOME/.pandoc` to
`$HOME/.local/share/pandoc`.
On Windows the default user data directory remains the same.
Closes#3582.
* Lua: allow access to pandoc state
Lua filters and custom writers now have read-only access to most fields
of pandoc's internal state via the global variable `PANDOC_STATE`.
* Lua: allow iterating through fields of PANDOC_STATE
* Lua filters doc: describe CommonState
* Lua filters doc: mention global variable PANDOC_STATE
* Lua: add access to logs
Log messages can currently only be printed, but not decomposed.
