Ability to create and see discussion in a merge request

Problem to solve

As a user, I want to see MR discussions inline when I open a file with discussions, so I can complete MRs more quickly.

Proposal

When I checkout a branch that has an active MR and I open a file that has discussion items in that MR, the editor should show a decoration indicating a discussion on those lines. Clicking on the decoration should open an inset panel showing the discussion. In this way, it would behave like unstaged stages on a VCS tracked file.

Further details

Links / references

(description provided by @firelizzard)

Original issue description

It would be great for code reviewing if we could add comments directly from VS Code when browsing merge request.