Amazon Q: Send message with Shift+Enter instead of just Enter

## Problem

I'd like to have an option to insert a new line with Enter key instead of sending a message. In CJK languages, the Enter key is used very frequently to confirm the input, and thus the current key binding often leads to an incomplete message mistakenly sent to QDev, which is really annoying.

For example, Slack also supports this option, which indicates it is a common demand for this kind of app.


Because many AI coding tools (e.g., Cline or Claude Code) do not support this option yet, this simple option can be a differentiator for QDev extension.

## Expected behavior

There is a global option for the key binding to send a message. Users can select Enter or Shift+Enter (or cmd+enter, whatever). If Shift+Enter option is selected, Enter key just adds a new line.

Start by locating the Amazon Q message input in the VS Code extension and the existing keybinding or settings entry points. Trace how Enter currently sends a message, then determine where a global Enter-versus-Shift+Enter choice belongs. Done means the selected binding sends or inserts a newline consistently, including the requested Shift+Enter mode.

Written by the indexing model from the issue text.