isModelsInstalled

Public because consumers need to know whether FreeMinecraftModels' model registry was invalidated during this startup. When this importer installs models it fires ModelInstallationEvent, which makes FreeMinecraftModels rebuild its registry asynchronously; a plugin that spawns modelled entities right after importing would find no models and silently drop them. Consumers check this flag to re-wait for FreeMinecraftModels only on the boots where an install actually happened.