fix(line-editor): Alt+D saves the killed word to the kill ring

What

Alt+D was bound to deleteWordForward, which removed the next word without pushing it to the kill ring — so Ctrl+Y could not yank it back. That is inconsistent with Ctrl+W (kill word backward, which does save) and with emacs/readline where Alt+D is "kill-word".

• Point Alt+D at the existing killWordForward (same logic, but saves to the kill ring).
• Remove the now-unused deleteWordForward duplicate.

Verification

• zig build clean (exit 0).
• PTY: in echo GONE, cursor before GONE, Alt+D then Ctrl+Y restores echo GONE → prints GONE.

One-line binding change + dead-code removal; no API changes.