[IDEA] Hook for Houdini layout worklet callback

[travisleithead]

As the format operation will trigger layout of the text, it will be symmetric with HTML, if calls to format can also trigger a layout worklet to run on the per-fragment level (synchronously, but off thread) during the API call, so that layout results have been modified on a per-line basis (as with HTML inline-level layout of the same) when the results are returned.