feat(niri): add workspace-taskbar support

[sewaustav]

Description:
This PR adds a workspace-taskbar mode to the niri/workspaces module, bringing functionality similar to the hyprland/workspaces module.
Motivation:
Currently, Niri users lack a native way to visualize windows and app icons within workspaces directly on the bar. While the wlr/taskbar module is an available option, it does not fully meet the needs for a structured taskbar; it often lacks sufficient organization of application icons and fails to maintain a logical or desired order, which is essential for efficient workflows on tiling compositors like Niri.

A native solution within niri/workspaces provides better integration, stable icon placement, and cleaner control over window management, specifically tailored for Niri's scrolling compositor.

Key Changes:

• Refactoring: I refactored the logic within the niri/workspaces module. The logic is now split into two entities: Workspaces (the controller/container) and Workspace (the entity/element). This allows for a more flexible rendering approach beyond simple button text.
• New Feature: Added workspace-taskbar support.
• Enables app icon display per workspace.
• Clicking an icon focuses the corresponding window.
• Middle-clicking an icon closes the window.
• Documentation: Updated man/waybar-niri-workspaces.5.scd with the new configuration options.

Testing & Compatibility:

• Backward Compatibility: Tested with existing configurations; no breaking changes detected.
• Stability: The module runs stably with no memory leaks detected during various usage scenarios.
• Icons: Verified compatibility with major icon themes, including Breeze, Adwaita, and Papirus. The module correctly retrieves and displays icons as expected.
  Configuration Example:

"niri/workspaces": {
    "format": "{icon}",
    "workspace-taskbar": {
        "enable": true,
        "icon-size": 18
    }
}