crossrefs: margin everything

[cscheid]

• ensure *-cap-location = "margin" works
  • HTML
    • it all works except table margin captions are centered in the margin which is ugly
    • custom floatreftargets work
  • PDF
    • [ ] tbl- and fig- works, but custom floatreftargets do not (sidecaption doesn't like being inside a custom float environment)
• ::: column-margin ... :::
  • HTML
  • [ ] PDF. The entire code path breaks with nested floats right now. We do slightly better wrt to warnings and lifting FloatRefTargets out of the divs, but the support is necessarily limited by what we can do with marginfigure and marginnotes interactions.

[cscheid]

We're going to have to close this without adding PDF support for custom crossreferences, because that's not really something we can do (marginfigure does something special for Figures and Tables)