MessageContextMenuRegistry

Maps each message context-menu command name to a presence marker. Populated by seedcord codegenseedcord codegen, so keyof MessageContextMenuRegistrykeyof MessageContextMenuRegistry is the compile-time union of every registered message context-menu name. Do not augment it by hand, run seedcord codegenseedcord codegen.

Kept separate from UserContextMenuRegistryUserContextMenuRegistry, see that interface for why.