syntax review: block-level editorial marks?

(cc @vezwork)

We would like block-level editorial markings:

[>> this is where you're wrong:

- a gish
- gallop
- of points]

But this runs afoul of a design principle we're trying to enforce. Unlike the Pandoc Markdown situation with footnotes, we don't support inline nodes that can contain blocks. qmd has, instead:

• footnote references
• footnote definitions in both inline and block form

That suggests that we could promote "footnote definitions" to be "ancillary definitions" that can serve as footnotes, or as the contents of editorial markings.

In hindsight, I think it's a bit unfortuante that we're avoiding blocks-within-inlines, because there's clear ergonomic value in having editorial marks that work at the block level as well. Specifically, it is a good thing to be able to select a bunch of blocks and a "suggest deletion" action that minimally changes the document lexically (for the purposes of automerge ergonomics)

Ok, so how about custom syntax support for block-level editorial marks?

::: >>

This is a long comment

:::

::: --

Delete all of.

These many, many paragraphs.

:::

The issue names no files, tests, or entry points. First resolve the syntax and design question around promoting ancillary definitions versus custom ::: block-level editorial marks, using the examples in the issue; done means an agreed syntax and implementation scope.