feat: Add configurable newline shortcuts with Ctrl+Enter as default

[0x23d11]

Description

Closes #4786

As described in issue #4786 , users need a more flexible way to insert newlines in the editor, especially when sharing code or asking questions about manually written code.

Solution

• Add UI configuration option to specify newline keyboard shortcuts
• Support ctrl+enter, and alt+enter for newline input

Checklist

• [] The relevant docs, if any, have been updated or created
• [] The relevant tests, if any, have been updated or created

Screenshots

In the following screen recording I'm using cmd+enter in mac to get a newline in the editor
https://github.com/user-attachments/assets/d24895da-665d-4a5a-b410-b043370ec14b

Testing instructions

[ For new or modified features, provide step-by-step testing instructions to validate the intended behavior of the change, including any relevant tests to run. ]

[netlify]

✅ Deploy Preview for continuedev canceled.

Name	Link
🔨 Latest commit	e5b6d91
🔍 Latest deploy log	https://app.netlify.com/sites/continuedev/deploys/67f52f6efeb38f00081d4460

[sestinj]

@0x23d11 Looking at the issue, it looks like that user wanted shift+enter to work, which should already work in Continue. If it doesn't, I think this is a separate bug that we should address rather than adding this additional complex code. I'm going to close this PR for now and maybe we can follow up with the person in the issue