Feature: Scrolling

[bhivam]

Right now if we have text that is too long vertically, it will go off the screen.

To implement scrolling, we should have a way of specifying the section of text we want to display and a way of updating this specification. The update will occur upon cursor movement.

Along with this, we want to be able to line numbers of 3 digits or higher.

To achieve this, maybe we should ask the calculate_content function to break up the text into a series of slices that each contain a full line of the overall content. This might make calculations easier.

While we're at it, we can change up the way the editor serves content to the frontend. Then we just add an update_content function which is called at the end of any content modification. This way we are not recalculating just to move the cursor around.