Add moving tabs left and right with key shortcuts

[vacenty]

#441 This PR adds option to move tabs with left and right with Ctrl+Shift+Left/Right Arrow or by using buttons in view menu.

[erik-balfe]

Reviewed against current master (cherry-pick conflicts in src/key_bind.rs and src/main.rs imports only; menu.rs + i18n merge cleanly).

Why it does not merge as-is

• key_bind.rs was refactored to delegate to ShortcutsConfig — new bindings belong in shortcuts.rs, not key_bind.rs.
• Ctrl+Shift+ArrowLeft/Right conflicts with existing split pane focus shortcuts on master (PaneFocusLeft / PaneFocusRight).
• TabMoveLeft / TabMoveRight also need wiring in shortcuts.rs (KeyBindAction, action_label, shortcut_groups, fallback_shortcuts) so they appear in View → Keyboard shortcuts and are user-remappable.

What looks good

• The TabMoveLeft / TabMoveRight handler logic using tab_model.reorder() is sound and matches the drag-and-drop path from feat: tab dnd #698.

Suggestion for defaults
Per #441, gnome-terminal-style Ctrl+Shift+PageUp / Ctrl+Shift+PageDown are free and avoid the pane-focus regression.

I'm preparing an updated PR rebased on master that incorporates this approach — happy to collaborate here or land separately. Thanks for getting this started!

[vacenty]

I've rebased changes. I've changed default shortcuts to the ones suggested by @erik-balfe as they seem more standard than the ones that I'm used to. In case adding moving tabs left and right to menu feels like too much I'm open to removing it.