Allow splitting of messages using `shift-enter` (#2613)

Closes
https://linear.app/zed-industries/issue/Z-2364/allow-splitting-blocks

Release Notes:

- Added support for splitting messages in the assistant panel using
`shift-enter`. (preview-only)
11f318566eNathan Sobo committed on 6/16/2023, 5:50:50 PM· committed by GitHubparents8235d78c179dd9
2 files changedLine totals unavailable