Display a indicator on the cursor's line

this is inspired by a Notepads feature - there's this little reveal-enabled box that can be drawn over the line with the cursor on it. It looks kinda cool. I don't know if I'd really use it, but it's a cool idea.

Probably makes more sense as an extension, so I'll tag it as part of #4000. Probably something that would be a good example of a _simple_ UI feature that an extension could provide.

Start by reviewing the referenced issue #4000 and the extension architecture, since this issue names no files, tests, or entry points. Clarify the cursor-line indicator's behavior and extension boundary with maintainers; done would be an agreed, working extension-based UI feature.

Written by the indexing model from the issue text.