Add inline completion hints (ghost text) to the editor

Show the common prefix extension of completions as dim ghost text after
the cursor. Press Right or End to accept the hint. Hints are suppressed
when the completion menu or search mode is active, and can be toggled
via *inline-hints-enabled*.

Inspired by the isocline library's hint system.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>