feat: syntax highlighting for gitlab duo chat

Description

This MR adds syntax highlighting (using highlight.js) to the Duo Chat messages.

Demo

before	after

Related Issues

Fixes #1435 (closed)

How has this been tested?

What CHANGELOG entry will this MR create?

• fix: Bug fix fixes - a user-facing issue in production - included in changelog
• feature: New feature - a user-facing change which adds functionality - included in changelog
• BREAKING CHANGE: (fix or feature that would cause existing functionality to change) - should bump major version, mentioned in the changelog
• None - other non-user-facing changes