[css-ui] Define how caret painting works (which clips apply to it, etc)

[emilio]

Gecko has historically painted the caret as regular in-flow content (both for purposes of z-sorting and clipping).

I've aligned it with WebKit-based browsers for compat in https://bugzilla.mozilla.org/show_bug.cgi?id=1663475, so that only clips from the containing block apply to it.

It'd be nice to have this defined somewhere. cc @MatsPalmgren

[frivoal]

If you have a definition taking form from implementation experience and compat requirements, I agree it could be nice to include it in the spec.