Adds auto scrolling for when dragging the selection handles. At one point this was working, but not for a long time - probably before we moved selection management from Rust to Swift.

This behavior is imperfect in that, if you drag a selection handle below the keyboard, the selection shows as the original sometimes until you jiggle it a little. How we stack up:

• Apple Notes automatically scrolls up when dragging selection handles but not down
• Obsidian scrolls down, and speed scales with how far down you drag it (fancy!), but scrolling up is always basically instant (probably a bug). Also suffers similar bugs as my implementation.

fixes #4000