Expose visual tab selection override in skin API

[takiguru]

What does this PR do?

Adds SetTabSelection(tab, selected) to the public skin API so addons can keep tab highlighting in sync with their own pages.

Passing true or false overrides the visual selection; nil restores native selection. This fixes cases where an addon tab and a Blizzard tab appear selected at the same time, without changing Blizzard’s tab state.

How was it tested?

In-game.

Screenshots

Checklist

• New settings default OFF (no behavior change without opt-in)
• Zero cost while disabled: no events registered, no polling, no hooks doing work, no frames built
• Cheap while enabled: event-driven (no polling, no timer-based logic, no per-frame allocations)
• No writes onto Blizzard-owned frames (weak-table pattern used); HookScript/hooksecurefunc only, never SetScript on Blizzard frames
• Tested in-game on live; no version gates or pre-Midnight APIs

Related PR: Adds EllesmereUI skin support to What's Training and uses this API to keep tab highlighting in sync: fusionpit/WhatsTraining#44