refactor: add Initial chat structure

Summary

This MR adds minimal file structure to make GitLab Duo Chat module available in our vscode extension. The module is hidden behind a feature flag. For the sake of reducing MR size and easier review process the module doesn't provide any useful functionality yet. More functionality will be added in a follow-up MRs.

This MR includes !897 (merged) . The diff should become smaller after merge of !897 (merged).

Related to &10960 (closed)

How to enable chat

• Enable chat feature flag via "config.gitlab.featureFlags.chat": true in your settings.json.
• Reload VSCode window with cmd+R.
• Open palette (cmd+P) and select "GitLab Duo: Open Chat" command.
• You should see sidebar chat window with "Chat Window HTML code." text inside.