Create editable fields within code snippets

After speaking with Landscape's product manager, he suggested we implement an editable field feature within the code snippet; which could update other pieces of code where the user's input may be needed. An example would be a tutorial where all the steps depend on a project's name, and they could replace that word in all following steps of the tutorial.

There is an example of this being implemented in [google's documentation](https://cloud.google.com/storage/docs/access-control/using-iam-permissions#gsutil_3)




Start by reviewing the linked Google documentation example and the existing code-snippet behavior in Vanilla Framework. Define how an editable value is represented, how updates propagate across later snippets, and which user interactions and tutorial cases must be covered before implementation.

Written by the indexing model from the issue text.