Add chat:newline as a rebindable action

Currently, inserting a newline in the chat input is hardcoded to Option+Enter (alt+enter) and cannot be rebound via keybindings.json.

Use case: Many applications use Shift+Enter for newlines and Enter to submit. Users coming from Slack, Discord, or other chat tools expect this pattern. I'd like to configure:

• Shift+Enter → insert newline
• Enter → submit (unchanged)

Current behavior:

• alt+enter inserts newline (not rebindable)
• Adding "shift+enter": "chat:newline" to keybindings.json has no effect because chat:newline isn't a valid action

Requested behavior:

• Add chat:newline to the available actions list so users can bind newline insertion to their preferred key