`move_line_down`

Moves all cursors 1 line down, removing any selections.
Repeating this will remember the vertical position of the cursors,
even when moving across shorter lines.

--- Examples ---

The cursor remembers its vertical position,
even after moving across the shorter line.
┌────────────────────────────┐     ┌────────────────────────────┐     ┌──────────────────────────────┐
│ This is a longer l[i]ne.   │     │ This is a longer line.     │     │ This is a longer line.       │
│ Shorter line.              │ --> │ Shorter line.[]            │ --> │ Shorter line.                │
│ This is another long line. │     │ This is another long line. │     │ This is another lo[n]g line. │
└────────────────────────────┘     └────────────────────────────┘     └──────────────────────────────┘