[css-anchor-position-2] Proposal: Insertion caret as an anchor

[vmpstr]

Anchor positioning is great for positioning things relative to other elements. Sometimes, I want to position things next to the insertion caret though, for things like autocomplete suggestions in a large contenteditable element.

I'm not sure if this has been proposed before, but I think this might be a good fit for anchor positioning.

/cc @tabatkins

[flackr]

This seems like it would be cleanest to do with a pseudo-element, e.g.

.my-editable-element::caret {
  anchor-name: --caret;
}

Though this idea of having a pseudo-element came up previously in #9707 (comment) and was resolved to stick to adding more caret- properties.