When syncing text to the browser IME mirror, we sometimes need to mark a
particular range as "editable". Native clients have a pull-based model,
so don't need this mechanism, but on the web we need to proactively push
content to the IME mirror `textarea`.

---

Release Notes:

- N/A or Added/Fixed/Improved ...

---------

Co-authored-by: zed-zippy[bot] <234243425+zed-zippy[bot]@users.noreply.github.com>