feat(sway/workspaces): Add CSS class to identify workspaces by number

[turbodt]

What does this PR do?
It exposes a CSS class to match sway workspaces' buttons by workspace index.

Related issues
Closes #

Checklist

• [ x] Code is formatted with clang-format
• [ x] Builds locally (ninja -C build)
• [ x] Man page updated for any new/changed user-facing option (man/)
• [ x] Tested against the affected module(s)

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge, with no concrete correctness, security, or repository-rule issues identified.

Summary

This PR exposes each Sway workspace's numeric index as a GTK CSS class and documents the new selector.

• Adds workspace-index-${index} to workspace buttons when they are created.
• Documents the selector in the Sway workspaces manual page.

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