feat: show changed files for the MR

This MR is the first feature allowing users to see details of an MR from within the VS Code. When you open an MR item in the GitLab TreeView, you'll see all files that have been changed in the MR. The following issue that's going to be implemented is #264 (closed) which will show the diff for each of these changes.

before	after

My plan is to not release this till the #264 (closed) is implemented, just to give the users a more wholesome experience.

Part of #263 (closed)