Enter in Insert in Vim Mode should insert newline

I'd expect when vim mode is enabled:

• When in insert mode
• pressing <enter> will insert a new line
• pressing <ctrl/opt-enter> will submit
• When in normal mode
• pressing <enter> will submit

This is similar to how other vim specific ai chats work (e.g. Avante in neovim)

Currently <enter> will always submit the chat, whether in insert mode or not