fix: consolidate mover class and monkeypatch isDragging

[rachel-fenichel]

Fixes #357

• Consolidates DragMover into Mover. I left the DragMover/Mover distinction in place in feat: introduce Mover actions + experimental DragMover implementation #352 to keep as close as possible to the original PR. Now that I'm working directly in movement code, I really just want one class instead of making coordinated changes in both.
• This follows feedback from @gonfunko in that review.

Fixes #359

• Patches isDragging in mover.ts

Tested by:

• Doing keyboard drags that end in an Esc
• Doing keyboard drags that end in an Enter
• Doing mouse drags after the fact to confirm that mouse dragging still works

[cpcallen]

This should have been per-workspace (and patching/unpatching handled carefully) because Mover can handle multiple simultaneous moves on one or more workspaces.