Improve mention visibility by adding a background color (#7014)

When the chat if going fast, It's hard to see who is mentioning you, so
this feature will make it more clear by the UI instead of needing to
read all the messages.

<img width="242" alt="Screenshot 2024-01-29 at 21 19 07"
src="https://github.com/zed-industries/zed/assets/62463826/65ec307d-5027-4ead-9568-854fc746c822">

Release Notes:

- Added background to messages that mention you.