Improve the marking algorithm editor

At the moment, the editing interface inside the question editor is just a plain text box you can write code in, and an area to try out different answers and see the resulting marking notes.

First of all, the editor should have tabs for each note, like the custom part type editor. That would allow separate inputs for the name, description and code.

Somewhere, it should show the format of the student's answer made available in the variable `studentAnswer`.

Because of the declarative way marking notes are calculated, it can be difficult to see how the marking algorithm runs in practice. Would it be worth showing the steps of the marking process after it runs? There'd be a step when a note is calculated, and for each feedback item.

Start by locating the marking algorithm editor and comparing its plain-text interface with the custom part type editor. Check how studentAnswer is exposed and how marking notes and feedback items are calculated; done would include note tabs, visible answer-format guidance, and a clear representation of the marking steps.

Written by the indexing model from the issue text.