Notify FragmentInstance of added/removed text (#35637)

Follow up to https://github.com/facebook/react/pull/35630

We don't currently have any operations that depend on the updating of
text nodes added or removed after Fragment mount. But for the sake of
completeness and extending the ability to any other host configs, this
change calls `commitNewChildToFragmentInstance` and
`deleteChildFromFragmentInstance` on HostText fibers.

Both DOM and Fabric configs early return because we cannot attach event
listeners or observers to text. In the future, there could be some
stateful Fragment feature that uses text that could extend this.