Hovering over the `(edited)` text inside a message displays a tooltip
with the timestamp of when the message was last edited:


![image](https://github.com/zed-industries/zed/assets/53836821/be6d68c2-7447-42bc-bd5e-7a9053b3c980)

---

Also removed the `fade_out` style for the `(edited)` text, as this was
causing tooltips to fade out as well:


![image](https://github.com/zed-industries/zed/assets/53836821/91d3cf6a-db58-4e1d-b257-663b2ce1aca4)

Instead it uses `theme().text_muted` now.


Release Notes:

- Hovering over an edited message now displays a tooltip revealing the
timestamp of the last edit.