fix(tui): Allow word backward and forward navigation in tui (via keybinds)

[MilesNorton]

https://github.com/sst/opencode/issues/1975

We can now pass in the following to the keybinds to allow for word navigation:

  "keybinds": {
     ....
    "word_backward": "ctrl+left",
    "word_forward": "ctrl+right"
     ...
  },