chat: fix autocompletion for usernames with dash (#7514)

Github usernames are allowed to contain `-`, but the autocompletion was
not working correctly.
We added `-` as an allowed character for markdown files. We are not
aware of any completions for markdown files, so this should be fine to
add.


Before:

![image](https://github.com/zed-industries/zed/assets/53836821/5b456ed6-3098-48e8-90db-f5f42b4aa535)

After:

![image](https://github.com/zed-industries/zed/assets/53836821/a544f465-0b68-46f5-9a15-83b4c755c3c0)


Release Notes:

- Fixed autocompletion for usernames with dash character in the chat
message editor

Co-authored-by: Remco Smits <62463826+RemcoSmitsDev@users.noreply.github.com>