FR: show conflict labels in the builtin merge editor (`jj resolve`)

[bryango]

Is your feature request related to a problem? Please describe.
The builtin editor is lightweight and convenient, but it's often quite confusing as it does not show conflict markers when used in jj resolve.

Describe the solution you'd like

• Since conflicts: add labels to conflicts #7692 conflicts are now rendered nicely with labels. It would be great if the labels are visible in the builtin jj resolve editor.
• An easy hack to achieve this is to present the conflict markers & labels as a - remove diff to the builtin diff editor so that users can easily drop them and at the same time select the desired lines.

Describe alternatives you've considered
Use external merge tools.

Additional context
Friendly ping @scott2000 for comments 😉