Follow up to https://github.com/zed-industries/zed/pull/56689. This PR
fixes an error where any instance of a slash character in the message
editor was going through the slash command validation. The fix makes the
validation get triggered only if the slash character is the very first
thing typed in the message. This follow the approach Slack, Discord, and
other apps that contain slash commands use. Also tweaked the messaging a
little bit so it's crystal clear what's happening.

Release Notes:

- N/A