One thing I did not address is how bookmarks can be deleted. While we have the command bm- to delete a bookmark, some editors make it so that you can delete a bookmark if you remove enough lines around the bookmark. I don't think this is a major issue, since I think that if a person bookmarks an area and deletes a line, they may come back to that same place.

However, just to be clear, right now, the bookmark does not move, so if you have a bookmark on line 15 and remove 10 lines around the bookmark, the bookmark will still be on line 15. I want to somehow make it so that the bookmark stays relative to the text it covers, but it seems difficult.
For example, if the editor was like this:

a
b
c <--- bookmark here
d
e

and I erase lines a and b, then the editor should look like this:

c <--- bookmark here
d
e

And the reverse would also be true if I add lines a and b back.