Add a walkthrough to the extension

[bra1nDump]

After installation the user should get a tutorial on how to use the extension. Let's use https://code.visualstudio.com/api/ux-guidelines/walkthroughs
And have a simple 3 step tutorial:

1. Showing how to create a @task comment and invoke the extension
2. Showing the feedback on the right side and how to cancel
3. Suggesting how to undo changes - cmd+z for each individual change