Integrates the Editor class into the MUD server's shell loop, allowing
players to enter and use the text editor from the game.
Changes:
- Add editor field to Player dataclass
- Modify shell input loop to check player mode and route to editor
- Add edit command to enter editor mode from normal mode
- Use inp (not command.strip()) for editor to preserve indentation
- Show line-numbered prompt in editor mode
- Pop mode and clear editor when done=True
- Add comprehensive integration tests
- Fix test isolation issue in test_movement_updates_position
