index: support backwards Selections
index: initial take at better behavior

Now, when a collapsed cursor is on a P below another P,
both within a BLOCKQUOTE, then only the bottom P would be
unwrapped from the BLOCKQUOTE.

Before, the entire BLOCKQUOTE itself would be removed, which
was annoying because it made it hard to "break out" of the blockquote
at that point.
index: remove unused `unwrap-node` dependency