How to get addMutationHook?
import addMutationHook from 'fontoxml-blueprints/src/addMutationHook.js'
Adds a mutation hook, which provides a way to react to changes in loaded documents independent of the operation making those changes. See the guide Create a mutation hook to get started, and check the concept page Mutation Hooks for more information and limitations.
The object describing the mutation hook which is to be registered.