Setting to toggle ENTER, SHIFT+ENTER shortcuts in chat UI

[JasonWeill]

Problem

I would like ENTER to send a message in the Jupyter AI chat UI, but it adds a new line.

Proposed Solution

Provide an option to switch the shortcut behavior.

When the option is disabled, ENTER adds a new line, and SHIFT+ENTER sends the message.

When the option is enabled, ENTER sends the message, and SHIFT+ENTER adds a new line.

Additional context

If #85 is fixed, the hints should reflect the current preference.

macOS UI conventions treat ENTER (typically on the numeric keypad) and RETURN (typically in the same row as the middle row of letters) as two separate keys. On Windows and Linux, even though the keys on the numpad and in the letter-key row send different scan codes, UIs tend to treat them identically.

Slack's "Advanced" preferences pane has options to change this behavior, and to handle code blocks bounded by ``` specially: