Rebuilds the list of registered SlashCo workshop addons by scanning every mounted addon for a lua/slashco folder, as well as legacy addons placed in the local addons/ folder.
Also detects the main gamemode's own workshop ID and mount state. Runs the SlashCo:RegisterAddons hook at the end, so other addons can register themselves manually via SlashCo.RegisterAddon.
Called automatically on load and whenever SlashCo.GameContentChanged runs.