Adjust IME action buffering to only apply to insert actions (#12702)

Follow up to https://github.com/zed-industries/zed/pull/12678
fixes https://github.com/zed-industries/zed/issues/11829

In this solution, we only buffer Insert Text actions from the macOS IME.
The marked text and unmark actions are eagerly processed, so that the
IME state is synchronized with the editor state during multi step
pre-edit composition.

Release Notes:

- Fixed an issue where the IME pre-edit could desynchronize from the
editor on macOS
([#11829](https://github.com/zed-industries/zed/pull/12651)).

Co-authored-by: Conrad <conrad@zed.dev>