Add Promote and Demote Commands

[vincode-io]

A feature from MORE and the outliner in Userland Frontier (both products by Dave Winer), the keyboard shortcuts (Command-[ and Command-]) which ‘promote' and ‘demote’ all the siblings below the current row. It probably sounds like a strange request, but it’s actually really useful and also very rare in outliners. It’s hard-baked into my muscle memory. Here’s an old documentation page from Frontier that mentions them: https://web.archive.org/web/20010210055314/http://frontier.userland.com:80/menus/outliner

From MORE’s manual:

Promote The Promote command moves all the subheads below a selected headline to the next higher level of the headline.

Demote The Demote command moves the headlines beneath and at the same level as a selected headline to the next lower level in the outline.