nicer `quarto preview` warnings from Lua

[cscheid]

TIL via @tarleb that PANDOC_STATE.log has a list of warning strings emitted by Pandoc. We could forward those to the front-end in quarto preview so they'd be more visible.

[tarleb]

I've implemented @cscheid's suggestion to provide access to the JSON representation of those messages

pandoc.read('\\foo{bar}', 'latex')
print(pandoc.json.encode(PANDOC_STATE.log[#PANDOC_STATE.log]))

{"column":10,"contents":"\\foo{bar}","line":1,"source":"","type":"SkippedContent","verbosity":"INFO"}