chore: move quickstarts to declarative shadow variables

[Christopher-Chianelli]

Description of the change

Change usages of VariableListeners and CascadingShadowVariable to declarative shadow variables.

Checklist

Development

• The changes have been covered with tests, if necessary.
• You have a green build, with the exception of the flaky tests.
• UI and JS files are fully tested, the user interface works for all modules affected by your changes (e.g., solve and analyze buttons).
• The network calls work for all modules affected by your changes (e.g., solving a problem).
• The console messages are validated for all modules affected by your changes.

Code Review

• This pull request includes an explanatory title and description.
• The GitHub issue is linked.
• At least one other engineer has approved the changes.
• After PR is merged, inform the reporter.

[Christopher-Chianelli]

Requires TimefoldAI/timefold-solver#1739

[triceo]

We're getting a little bit ahead of ourselves here.

To put DSV in quickstarts, they first need to be promoted to public API.
To do that, there are still some outstanding issues.

No point in merging this before those happen, as this will all change anyway. (No preview, different imports, maybe even different names.)