TUI keybinds

jj-slice will let the user operate on the boundaries between changes. I'm trying to match some semantics from typical JJ tooling while also integrating some niceties from Helix:

• j/k: move up/down one position
• J/K: move to the next nearest boundary up/down
• Ctrl+j/k: on a boundary, drag it up/down one position
• gg/ge: go to the first/last position in the range
• gw<short ID>: go to the position before (after?) the given change ID (configurable?)
• space: toggle boundary at position
• enter: toggle boundary at position and move down one position
• b: set explicit bookmark for chunk
• a: toggle all positions consistently
• c: confirm selection
• q: quit
• ?: show help