Add a chat:newline bindable action for multi-line input

Currently there's no way to insert a newline in the chat input field via a keybinding. The available actions don't include a newline/line-break action, so users can't bind e.g. Shift+Enter to insert a newline.

The only workarounds are using an external editor (Ctrl+G) or pasting multi-line text.

Request: Add a chat:newline action (or similar) that can be bound to a key in keybindings.json, so users can configure a shortcut like Shift+Enter to insert a newline without submitting.