Add debug state for editor

[jorg-vr]

I encountered a small issue when running a simple input/output exercise (sum of 2 numbers): the input of the test was correctly loaded into the input area, but the lines were marked as checked from the start.

this would also solve #611

Some other suggestions, probably for a later PR:

• If you click debug, wait for the traces to generate and then edit the code you are still debugging the old code. In addition, there is no good way to make the code arrow go away. I would enter a "debug mode" when clicking debug which restricts some features to make this more clear (show stop button, read only editor).
• The "code arrow" doesn't scroll into view when it's not visible. Maybe we can reuse the active line feature of the editor?

Originally posted by @bmesuere in dodona-edu/dodona#5336 (review)