feat: add /fix command for GitLab Duo chat

Description

Adds GitLab Duo /fix command to VSCode:

• inside the Duo Chat webview
• as a command in VSCode

Related Issues

#1456 (closed)

How has this been tested?

• launch extension host
• add some example code with problems to fix
• highlight the code

then:

• ctrl+shift+p to open command pallete
• run GitLab Duo: Fix
• Duo Chat opens and runs /fix on the selected code

or

• open Duo Chat
• type /, verify /fix is an option
• using it runs the /fix command on the selected code.

Screenshots (if appropriate)

Duo Chat webview	VSCode command palette

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

Closes #1456 (closed)