Feature Request: Keyboard shortcuts for moving lines

### What
It would be very helpful to have keyboard shortcuts to move a line up or down. These exist in the electron Simplenote apps (alt + up/down, see https://github.com/Automattic/simplenote-electron/issues/1001), but the same shortcuts do not wok on the macOS app.

### Why
Having this would make managing to-do lists much easier.

Start by reviewing the macOS editor's keyboard handling and compare the requested behavior with the referenced Electron issue. Done means Alt+Up and Alt+Down move the current line up or down in the macOS app, matching the existing Electron shortcuts.

Written by the indexing model from the issue text.