If you use a custom syntax definition that refers to a syntax
you haven't loaded, pandoc will now complain when it is highlighting
the text, rather than at the start.
This saves a huge performance hit from the `missingIncludes` check.
Closes#4226.
This version fixes a bug that made it difficult to handle failures while
getting lists or a Map from Lua. A bug in pandoc, which made it
necessary to always pass a tag when using MetaList or MetaBlock, is
fixed as a result. Using the pandoc module's constructor functions for
these values is now optional (if still recommended).
