Merge Editor: Add a single line diff view on 3-way-merge

<!-- ⚠️⚠️ Do Not Delete This! feature_request_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- Please search existing issues to avoid creating duplicates. --> <!-- Describe the feature you'd like. --> Beyond compare 4 has a small header above the result pane, that shows the selected line from the 3 merged versions (base, current, incoming), and highlights the changes. This is very useful when trying to inspect the changes between them. Example:  Notice that in the side-by-side diff it's harder to notice the exact changes in each side, while the single-line view utilizes the whole window width, and it's easier to compare them. On this example, you can notice that Current (the left side, top row, light-blue arrow) added `, [BotFramework]` and did *not* delete the parenthesis, while the Incoming (right side, bottom row, purple arrow) added `, [InboundTeamsConnAmount],` and *did* delete the parenthesis (it moved to the next line). The changed parts are also highlighted. It would be very nice to have this kind of diff in the merge editor.