Custom writers can define a default template via a global `Template`
function; the data directory is no longer searched for a default
template.
Writer authors can restore the old lookup behavior with
``` lua
Template = function ()
local template
return template.compile(template.default(PANDOC_SCRIPT_FILE))
end
```
This ensures that the golden files in `test/fb2/reader/` don't
have newlines converted. This should fix a test failure on
GitHub CI with Windows.
Closes#5747.