Add a send button to the assistant (#12171)

![CleanShot 2024-05-22 at 18 20
45@2x](https://github.com/zed-industries/zed/assets/1789/dac9fcde-9fcb-4c40-b5da-ebdc847b3962)

Include the keybinding to help people discover how to submit from the
keyboard.

I'm also shifting from the word "Conversation" to "Context".

Release Notes:

- Added a send button to the assistant panel.