How to get externalDataManager
import externalDataManager from 'fontoxml-templated-views/src/externalDataManager.js'
Some templates need to render something not in the current document, or something that is derived from other data, which is expensive to compute. To be able to react to changes in this data, external data can be used in a template.
Set a piece of external data and cause a rerender for any templated views depending on that data.
Note that internal mutations in the given value will not trigger view updates. It is therefore recommended to keep external values immutable and/or as flat as possible.