GitLab Chat: Add ability to change height of input

What does this MR do and why?

Adds the ability to resize the AI input field to being able to put larger questions in (e.g. code blocks).

• Auto increases the height until 240px
• Adds scrolling beyond 240px
• Shift + Enter will add a new line
• Enter will submit

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Help > Ask GitLab Chat
2. Enter a long question
3. Verify input field

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.