moveCursor

pub fn moveCursor(self: *Selection, idx: usize, select: bool) void {
        //std.debug.print("moveCursor {d} {}\n", .{ idx, select });
        self.affinity = .after;
        if (select) {
            if (self.cursor == self.start) {
                // move the start
                self.cursor = idx;
                self.start = idx;
            } else {
                // move the end
                self.cursor = idx;
                self.end = idx;
            }
        } else {
            // removing any selection
            self.cursor = idx;
            self.start = idx;
            self.end = idx;
        }

        self.order();
    }