Add chat:newline keybinding action for multi-line input

Currently there's no way to insert a newline in the chat input via keybindings. The only multi-line option is ctrl+g to open an external editor.

Many chat applications support shift+enter or option+enter to insert a line break while enter submits. The keybindings system supports remapping chat:submit to a different key, but there's no corresponding chat:newline action to assign to a key.

Request: Add a chat:newline action (or similar) to the Chat context so users can configure a key combination to insert a newline without submitting.

Example use case:

{
  "context": "Chat",
  "bindings": {
    "alt+enter": "chat:submit",
    "enter": "chat:newline"
  }
}