feat: add focus_cross_monitor_mru to focus MRU window on cross-monitor move

[lefrogfrog]

Summary

Adds a new focus_cross_monitor_mru config option. When enabled, directional
focus crossing a monitor boundary (and focusmon) focuses the most recently
used window on the target monitor instead of the geometrically closest one.

Changes

• config: new focus_cross_monitor_mru option (src/config/parse_config.h)
  — struct field, parsing, clamping (0/1), default 0.
• dispatch (src/dispatch/bind_define.h):
  • focusdir(): when the option is set and direction-selected client is on a
    different monitor, delegates to focusmon(arg).
  • focusmon(): picks the MRU window (focustop) when the option is set or no
    explicit client target is given; cursor-warp handling reworked so it follows
    the focused client.
• docs: document the new option in docs/configuration/miscellaneous.md.
• .gitignore: ignore compile_commands.json and .clangd.