Fix ItemRack failing to load and set hotkeys on current Classic Era

[JSchott3]

Summary

Three issues traced to recent Classic Era client changes that together left ItemRack completely non-functional (no buttons, no bindings) or with hotkeys registered but not firing:

• GetAddOnMetadata/LoadAddOn/EnableAddOn/DisableAddOn/IsAddOnLoaded were removed as globals in favour of C_AddOns.*. ItemRack.lua's very first executable line called GetAddOnMetadata unconditionally, so the whole file errored out immediately on load (attempt to call a nil value) and nothing in the addon ever initialized.
• SetSetBindings() required GetCurrentBindingSet()'s return value to also be present in Enum.BindingSet before calling SetBindingClick. That membership check never passes on current Classic Era, so it silently retried a few times and gave up without ever binding any set hotkeys.
• The per-set hotkey macro mixed an insecure /script ItemRack.RunSetBinding(...) line with protected /equipslot [combat]... commands in the same macro body. That combination is silently vetoed, so even once a binding was registered and visible in the Key Bindings UI, pressing the key did nothing. Moved the Lua call to a PostClick handler (the same pattern the addon's other buttons already use) and left only the protected commands in the macro text.

All three were verified against a live 1.15.9 (interface 11509) Classic Era client — reproduced each failure, confirmed the fix resolves it, and confirmed gear-set hotkeys work end-to-end afterward (registration, live binding, and actual keypress).

Test plan

• Fresh login with the patched addon: no load errors, buttons appear and can be added via Alt+Click as before
• Set a gear-set hotkey (F1-F6), confirmed it shows in Key Bindings and GetBindingAction reflects it
• Pressed the hotkey in and out of combat, confirmed the gear set actually equips