How to get CustomMutationResult?
import CustomMutationResult from 'fontoxml-base-flow/src/CustomMutationResult.js'
Represents the result of a custom mutation.
Set the active property of the operation using this custom mutation.
Indicates the custom mutation can not be applied in the current situation.
Any changes made to the blueprint or selection will be ignored.
Indicates the changes in the blueprint and/or selection should be applied to the document, provided the result is valid.