[plt-scheme] Advice on adding a line numbers and/or a gutter to the editor

I would like to prepare a tool to add a gutter to the editor. A gutter
is an area that would appear on the left side of the screen in which
things "interesting" to the user could be placed.

For example, line number if one desired. Another thing that interests
me is the ability to click on the gutter to leave bookmarks. I could
even add comments in the bookmarks themselves, along with links to
other bookmarks. The idea is that later on I could browse my bookmarks
to go to places about which I cared for some reason.

Perhaps those are two separate tools, line numbers and gutters.

Where should I start looking or about what should I care learning to
get started on the path towards adding line numbers?