VSCodeExtension/DuoChat: Implement /fix Slash Command context menu and in chat drop down list

With the recent addition of the /fix slash command to the DuoChat API, we need to update the VSCode extension to include this new command. This update will ensure that users can access and utilize the /fix command directly from the VSCode interface, similar to other existing slash commands.

This includes:

/ command preview

secondary click context menu