Markdown is used as is in online documentation and in cli the markdown decoder
is used to decode and the some jq code massages it into something cli friendly.
Was just too much of a mess to have doc in jq.
json, yaml, toml, xml, html, csv are now normal formats and most of them also particiate
in probing (not html and csv).
Also fixes a bunch of bugs in to/fromxml, to/fromjq etc.
Now repl, slurp and help implemented using same query rewrite.
Include filename context in error if possible.
Add spew function that does opposite of slurp.
Start of help infra, not done or documented yet.
Show error pointer on parse error.
Rename internal eval to _eval and make eval be wrapper that
does rewrite and has various eror handling etc.
Nicer repl, slupr and help errors.
