Frozen code: show all the code students need to understand what's happening, but frozen

[NathanLovato]

It's all in the title. This issue supersedes #72 and intends to solve the issue underlined by #758 and other issues in the past.

We have known we need this feature since the beginning: the ability to show "frozen" bits of code (code that the student can read and select but not edit) and possibly foldable code.

We want to be able to show:

• How the student's code gets executed (as mentioned in #72)
• Code or values the student is dealing with. Even if they can't understand all of it.

It's something tricky to achieve in Godot and generally a feature we want to add in v2 of the course and app, which should use a differently technology stack.