Custom AST can respond to "aspects"

[cscheid]

Because we have full control over AST processing now, we can make custom AST nodes respond to "aspects". Let me explain.

We already have the ability to visit "any" custom node by adding a Custom entry to the filter.

But this trick is probably more generally useful. Imagine if a node could register with quarto that it will respond to CanHold (so that in latex formats, it knows to render itself in a way robust to outer par issues). Then, nodes that need their contents to hold can just write a filter that sets all CanHold callers correctly.

[cscheid]

cc @dragonstyle.

(also, see src/format/pdf/format-pdf.ts)