Add support for dynamic text in bubbles

[NathanLovato]

Currently, we cannot have code evaluated while the tour is running in text bubbles. It means we can't provide feedback to the user, like showing where a node is placed, or dynamically tell them which node is selected, which could slightly improve the user experience.

This low-priority task is about adding a way to show text evaluated when the user is on a given step.

This should account for:

• Storing data or text in a previous step and displaying/accessing it later.
• Changing the bubble text based on some expression or function evaluated when running the step.
• (optional, to consider) Changing the text in realtime during a step, based on a signal or a function run every frame. This would be to give instant feedback on what the user is doing, to assist them in completing a task, for example.